Sciencemadness Discussion Board

Metal displacement of aluminum by a copper salt

SnailsAttack - 24-3-2022 at 18:51

Simple question: how do copper salts displace aluminum metal? Like, if I soak aluminum scrap in copper acetate it starts bubbling and the solution turns clear within 5 minutes. How is it penetrating the aluminum oxide passivation layer? Most other oxides seem highly resistant to metal displacement reactions; notably those of titanium, iron, and manganese.

Rainwater - 25-3-2022 at 02:03

Simple answer

For stp conditions in aqueous solution, you want to review the reactivity series.
https://en.m.wikipedia.org/wiki/Reactivity_series
Quote: Originally posted by SnailsAttack  
How is it penetrating the aluminum oxide passivation layer?

This layer is not perfect. If you were using anodized aluminum, it would be very resistant to this attack, taking much longer to react.

Al + 2H2O = H2 + Al(OH)2
As soon as the aluminum surface reacts with water, al ions are formed. This is a white powder that is not very soluble. Iron is very reactive to. Just look for some rust. titanium is as well, the oxide layer formed is very resistant to attack.

In solution, you have aluminum, water, and copper salt.
Assuming your at room temp, and 1 atm of pressure, you can use the stp table linked above.
Al+ H+ OH- Cu+ SO4-
Let's ignore the water for simplicity, which leaves Al+ Cu+ SO4-
Because copper is lower down on the table, it will accept an electron from aluminum
Now you have Cu Al+ SO4-

You would be much better off reading this out of a textbook than from me. The reaction mechanisms are much more complex in reality. Research gibbs free energy for a more accurate method of perdicting the products of a reaction.

SnailsAttack - 25-3-2022 at 08:27

Quote: Originally posted by Rainwater  
Simple answer

For stp conditions in aqueous solution, you want to review the reactivity series.
https://en.m.wikipedia.org/wiki/Reactivity_series

Quote: Originally posted by SnailsAttack  
How is it penetrating the aluminum oxide passivation layer?

This layer is not perfect. If you were using anodized aluminum, it would be very resistant to this attack, taking much longer to react.

Al + 2H2O = H2 + Al(OH)2
As soon as the aluminum surface reacts with water, al ions are formed. This is a white powder that is not very soluble. Iron is very reactive to. Just look for some rust. titanium is as well, the oxide layer formed is very resistant to attack.

In solution, you have aluminum, water, and copper salt.
Assuming your at room temp, and 1 atm of pressure, you can use the stp table linked above.
Al+ H+ OH- Cu+ SO4-
Let's ignore the water for simplicity, which leaves Al+ Cu+ SO4-
Because copper is lower down on the table, it will accept an electron from aluminum
Now you have Cu Al+ SO4-

You would be much better off reading this out of a textbook than from me. The reaction mechanisms are much more complex in reality. Research gibbs free energy for a more accurate method of perdicting the products of a reaction.

Oh, I'm familiar with the MRS I just wasn't sure how the copper acetate was getting past the oxide layer (or otherwise if it was reacting with the aluminum oxide itself).

So you're saying it's because it can get through the thinner gaps in the layer?

reactofurnace - 25-3-2022 at 09:26

Quote: Originally posted by Rainwater  
Simple answer

For stp conditions in aqueous solution, you want to review the reactivity series.
https://en.m.wikipedia.org/wiki/Reactivity_series

Quote: Originally posted by SnailsAttack  
How is it penetrating the aluminum oxide passivation layer?

This layer is not perfect. If you were using anodized aluminum, it would be very resistant to this attack, taking much longer to react.

Al + 2H2O = H2 + Al(OH)2
As soon as the aluminum surface reacts with water, al ions are formed. This is a white powder that is not very soluble. Iron is very reactive to. Just look for some rust. titanium is as well, the oxide layer formed is very resistant to attack.

In solution, you have aluminum, water, and copper salt.
Assuming your at room temp, and 1 atm of pressure, you can use the stp table linked above.
Al+ H+ OH- Cu+ SO4-
Let's ignore the water for simplicity, which leaves Al+ Cu+ SO4-
Because copper is lower down on the table, it will accept an electron from aluminum
Now you have Cu Al+ SO4-

You would be much better off reading this out of a textbook than from me. The reaction mechanisms are much more complex in reality. Research gibbs free energy for a more accurate method of perdicting the products of a reaction.


Well said

Rainwater - 25-3-2022 at 09:28

Yes/no basicly.
The reaction of vinager on aluminum metal is faster than on aluminum oxide.
Both reactions create the same water soluble salt.

Fulmen - 25-3-2022 at 13:42

The oxide layer is always slightly porous and single ions are pretty small. The rest is, as the french say, c’est la vie.