Investigating Metal Activity Differences

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Investigating Metal Activity Differences

Intro:

Pennies minted prior to 1982 contain an alloy of copper and zinc, while those minted after 1982 contain mostly pure zinc covered with a thin layer of pure copper. The analysis is based on the differing reactivities of zinc and copper with hydrochloric acid. In hydrochloric acid, zinc reacts following the reaction of:

Zns + 2HClaq → ZnCl2aq + H2g

While copper does not react with hydrochloric acid

Cu + 2HCl → CuCl2 + H2        No Reaction

The procedure will involve cutting a penny into quarter-sections to expose the zinc core to the hydrochloric acid.  We will weigh these sections and the place them into concentrated hydrochloric acid. The zinc will mostly likely dissolve away, leaving the copper shell behind because copper does not react with hydrochloric acid. The dried copper shell is weighed and from its mass and the initial mass of the sample the percent copper can thus be calculated:

percent Cu = mass of Cu      _              x 100

                  mass of penny

Purpose:

        The purpose of this lab will be to determine the percentage of the less expensive zinc used as a filter in the US pennies made after 1983 versus the percentage of the almost pure copper used previous to 1983.

Procedure:

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  1. Obtain pennies that are minted after 1983 and cut them into half-sections using metal shears.  Note the silver-colored zinc core and the outer copper layer
  2. Record the mass of two of the cut penny sections to the nearest 0.001 g.  Place them into a labeled 100 ml beaker.
  3. Repeat step 2 with the other remaining half-section.
  4. To each beaker, add about 20 mL of concentrated (12 M) hydrochloric acid.  When gas bubbles are no longer visible, the zinc has completely dissolved
  5. Discard the acid in the sink once the bubbles have stopped appearing by filtering the copper particles ...

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