Dissolving the Oxide Layer of Aluminum

Aluminum metal is very reactive, and yet is resistant against a wide range of chemicals. Some chemicals that it is resistant to include acids, reductants, ionic compounds, some bases and oxidisers. This is because aluminum has a very thin oxide layer that forms very quickly in our oxygen rich atmosphere. Anodized aluminum has a even thicker layer, built up using electrochemistry. There are only a few chemicals that can quickly destroy the oxide layer, mercury and alkali hydroxides to name two. If mercury is applied to a piece of aluminum, it will corrode away very quickly due to mercury disrupting the oxide layer. This is why mercury and mercury compounds are forbidden on airplanes. There is a few more chemicals which can quickly attack aluminum, one of them is the tetrachlorocopper anion. The tetrachlorocopper anion can be formed from any soluble copper salt and any soluble chloride salt or hydrochloric acid.

2 HCl + CuCl2 ---> H2(CuCl4)

2 Al + 3 H2(CuCl4) ---> 2 H(AlCl4) + 4 HCl + 3 Cu

2 Al + 6 H2O ---> 2 Al(OH)3 + 3 H2

Warnings:
All copper compounds are moderately toxic.
Hydrogen gas is formed in this reaction, which is extremely flammable and can explode.
Hydrochloric acid is corrosive and produces heat when mixed with water.

Materials:
31.45% hydrochloric acid (pH down for pools)
Copper (II) chloride (Any copper compound can be substituted)
Aluminum foil

September 21, 2005
A stoichiometric amount of copper (II) chloride and hydrochloric acid were combined, the solid turned black and the solution a dark green. Enough water was added to dissolve the reagents. A grey-white substance was left undissolved.

A few drops were put on a piece of aluminum foil and it bubbled vigorously, liberating hydrogen, steam and leaving behind a copper powder.

A graduated cylinder was filled with distilled water and inverted into a dish full of distilled water without letting any air into the cylinder. 0.12g of aluminum foil was placed in the cylinder. The prepared solution was forced into the top of the cylinder with a pipette. Water is also removed from the dish so it doesn't overflow. A few seconds after the concentrated solution was added to the cylinder, small brown hair-like growths appeared. About 15 minutes later the aluminum was covered in mossy copper deposits. Hydrogen was continuously bubbling off. This was continued for approximately 40 minutes until all aluminum was consumed. 39.6 mL of hydrogen gas was collected in the graduated cylinder.














Unanswered Questions:
By what mechanism does the tetrachlorocopper anion destroy aluminum oxide?

The text presented here is for informational purposes only. The author is not liable for actions taken by the reader.