Chemical Composition of American Coins

O'Connell, Lawrence P.                   Lincoln Park High School
                                         280-3600
                          

Objectives:
 
Students will:
1. understand the role that chemistry plays in identifying the 
   composition of coins and in determining suitable substitutes if 
   these compositions have to be changed. 
2. understand a metals activity chart and its implications. 
3. understand the Law of Conservation of Matter and Energy and how the 
   balancing of equations relates to it.                                 
4. become familiar with physical and chemical properties and their use 
   in identifying substances.
5. understand the role electrons play in the formation of ions and 
   compounds.
6. understand the nature of ions and how they differ from elements or 
   compounds. 

Apparatus Needed:

beaker, 100 mL
forceps
triangular file
U.S. penny-1983 or later

Recommended Strategy: 

The 1983 and later dated pennies are comprised of a zinc core covered 
with copper.  They are 97.5% zinc and 2.5% copper. Pennies dated 1981 
and earlier do not have a core.  Both types of pennies were minted in 
1982 and 1983.  It is necessary to scratch the edge of the penny with 
a file so that the zinc core can come in contact with the acid. 

Various strategies can be used in doing this lab.  One suggested 
strategy is to acquaint students with the activity relationship that 
exists between metals (specifically copper, hydrogen and zinc) and let 
them determine the equation that represents the reaction.

1.  CAUTION.  Wear goggles and apron.  You will be working with a 
    strong, corrosive acid.
2.  Using a triangular file make 10-12 scratches around the edge of 
    the penny.
3.  Use a forceps to CAREFULLY place the penny into a 100 mL beaker. 
    containing 25 mL of 6.0 M hydrochloric acid.
4.  Leave the reaction undisturbed overnight.
5.  When the penny is floating, carefully remove it with the forceps.  
    rinse it in a container of water, carefully examine it and note your 
    observations. 

Questions:

1.  What is the balanced equation that shows the reaction that took 
    place? 
2.  Why can't you see the zinc after the reaction? 
3.  What causes the penny to float? 
4.  Trace the changes in the element hydrogen before, during and after 
    the reaction.
5.  Dependent on their ability in the field of stochiometry, students 
    could be assigned various problems in this field various problems 
    connected with the reaction.