Microscale chemistry with a coin and a can
Mahmoud K. El-Marsafy, Peter Schwarz

3. Electron transfers from activated aluminium to water molecules

..
  Photo 3.1 - 3.3
A Pfennig coin activates a can closure to react with water

1: A can closure - separated by a piece of transparent plastic – is placed on a Pfennig coin. The two metals are connected by a crocodile clamp. 2: This metal combination stands in a small honey packaging with concentrated table salt solution. 3: After some hours a white gelatinous precipitate is observed in front of the can closure (Al) as well gas bubbles at their rims.
The two metals in the salt solution act as a Galvanic Cell.
The observations are the result of a direct electron transfer from aluminium to water. A contact corrosion of aluminium takes place.
[Beside this reaction an indirect electron transfer from aluminium via coin as cathode of the Galvanic Cell is postulated, because gas bubbles (Photo) can also be observed at the rim of the Pfennig coin if the table salt solution is replaced by 0.5 M sulfuric acid.]
 Reduction:        6 H2O(l) + 6 e- --> 6 OH-(aq) + 3 H2(g)

[6 H+(aq) + 6 e- --> 3 H2(g)]
Oxidation:                      2 Al(s) --> 2 Al3+(aq) + 6e-
Redox reaction:   6 H2O(l) + 2 Al(s) --> 2 Al(OH)3(s) 
last modification: 07.10.01         back

 

 . . .
   . 

back