Floating Penny (Demonstration)
(Reference: Borgford, C.L., and Summerlin, L.R.  Chemical Activities; American Chemical Society: Washington, D.C., 1988; pp 77-78.)
 

Metal/acid displacement   :   Activity series
 

The whole class can do this!

Purpose:
Illustrate the idea of metal displacement reactions
 

Safety:
Safety goggles should be worn by instructor and students.  HCl is corrosive; handle with care.
 

Materials:
Pre-1982 pennies (# of groups)
Triangular file
Post-1982 pennies (# of groups)
Forceps
150 mL 6 M HCl
100 mL beaker (# of groups)
 

Procedure:
1. Use a file to score the pennies- about four 1mm scores
2. Place 1 of each type of penny in beaker
3. Cover with about 20 mL acid
4. Leave this in fume hood until next day, post-1982 penny should be floating and the pre-1982 penny should still be on bottom of beaker
5. Remove pennies and rinse thoroughly

 
Explanation:
The reason for the differences in the years, is that instead of being pure copper, the post-1982 pennies contain zinc in the center, and this will react with HCl because Zn2+ has a lower reduction potential than H+.

Zn(s)  +  HCl(aq)      H2(g)  +  ZnCl(aq)
 

Clean-up:
Can use pre-1982 pennies; post-1982pennies should be discarded in trash once all of the acid has been rinsed.  The ZnCl solution can be rinsed down drain with lots of water.
 
 

Ginger Chateauneuf, 2000.
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