Aluminum Properties: Why Does It Not React with H2 and Plate Silver?

[kq6up]

I find it strange that Aluminum is high on the activity series but does not react with evolution of hydrogen with many strong acids. I actually tried this when I was a kid. I had access to any acid or salt I wanted. I tried dissolving Al in H2SO4, HNO3, and HCl with no H2 bubbles. Aqua regia absolutely destroyed it. Is this due to the oxide coating on the Al?

Moreover, I have read that it is inert to single replacement of Ag in spent photographic fixer, and that it is recommended to use as an electrode to plate Silver out of spent fixer. I have it in old photographic fixer now, and it does not seem to be involved in any replacement reaction.

Cu would not work as it would be replaced by the Ag out of the fixer, and it would just cause the Ag to precipitate instead of plating out.

Any hints as to what is going on hear would be appreciated.

Thanks,
Chris Maness

 

[Alkim]

Al is indeed very reactive. However, usually it is covered by a thin and compact oxide layer. If you remove that layer and assure it is not formed immediately (e.g. rubbing the surface of the metal with mercury or mercury nitrate), the metal behaves more or less like magnesium, reacting vigorously with air, water and acids. Also, aluminum powder with no previous treatment burns like magnesium when thrown into a flame.

 

[Borek]

http://en.wikipedia.org/wiki/Passivation_(chemistry)

 

[kq6up]

It is working. There is grey Ag on the bottom with a Ag colloid hovering above it. The Al electrodes was inert in this process. They look exactly the same.

I think I would rather plate it out than gather a precipitate. Al would be an ideal substrate because it does not dissolve in HNO3 readily. However treating the surface might change that.

My desire is to convert all of this to AgNO3 to experiment with wet plate or tin type photography. I have five gallons of very heavily used fixer. It probably contains well over an ounce per gallon.

Thanks,
Chris Maness

 

[LudusRex]

I can provide some insight into why aluminum does not react with H2 and plate silver. First, let's discuss the property of reactivity and how it relates to the activity series. The activity series is a list of elements organized from most reactive to least reactive. This means that elements at the top of the list are more likely to undergo chemical reactions compared to those at the bottom.

Aluminum is high on the activity series because it is a highly reactive metal, but this does not mean it will react with all substances. In the case of H2, aluminum has a strong affinity for oxygen, meaning it will readily react with oxygen to form aluminum oxide (Al2O3). This oxide coating on the surface of aluminum acts as a protective layer, preventing the metal from further reactions. This is why you did not see any H2 bubbles when you tried to dissolve aluminum in strong acids.

In the case of silver plating, aluminum is used as an electrode because it is more reactive than silver. This means that it will readily give up electrons to the silver ions in the spent fixer, causing the silver to be plated onto the aluminum electrode. This is known as a single replacement reaction, where a more reactive metal replaces a less reactive metal in a compound.

You mentioned that copper would not work in this scenario because it would be replaced by the silver and cause it to precipitate. This is because copper is less reactive than silver and would not be able to give up electrons to the silver ions.

In summary, the lack of reaction between aluminum and H2, as well as its ability to plate silver, can be explained by its reactivity and the presence of an oxide coating on its surface. I hope this helps to clarify the properties and behavior of aluminum in these situations.