CHEM 1102

Pennies From Heaven

Purpose: There are three purposes to this experiment: 1) to perform a quantitative analysis of a penny, 2) to observe some chemical properties of copper and zinc and 3) to collect data on the masses of pennies minted in different years.

Introduction:

Part 1, Analysis of a penny: Pennies minted prior to 1982 are composed of an alloy of copper and zinc, whereas those minted after 1982 contain a core of nearly pure zinc covered with a layer of pure copper. In Part 1, the percentage of copper in a post 1982 penny will be determined. The analysis is based on the differing reactivities of zinc and copper with hydrochloric acid. In hydrochloric acid zinc reacts according to the reaction:

whereas, copper does not react with hydrochloric acid.

The procedure involves cutting a penny into sections to expose the zinc core, weighing the sections and the placing them into concentrated hydrochloric acid. The zinc dissolves leaving the copper shell behind. The dried copper shell is weighed and from its mass and the initial mass of the sample the percent copper can be calculated.

percent Cu =

Part 2, Chemical properties of Cu and Zn:The net ionic equation for reaction 1) is

Zn(s) + 2H⁺(aq) ® Zn²⁺(aq) + H₂(g)

This is an oxidation-reduction reaction, in which zinc metal is oxidized by hydrogen ion. Although hydrogen ion oxidizes zinc it is not a strong enough oxidizing agent to oxidize copper. However copper is oxidized by nitric acid.

Cu(s) + 2NO₃^- + 4H⁺ ® Cu²⁺(aq) + 2NO₂(g) + 2H₂O(l)

The oxidizing agent in this case is the nitrate ion, a stronger oxidizing agent than H⁺:

Part 3, Masses of pennies versus date: The masses of several pennies will be determined, and the variation with mint date examined. The data obtained by the entire class will be pooled and analyzed at a later date using a computer spreadsheet program.

Stockroom: Things to borrow and return on the same day.

• Bag of 10 mixed pennies (do NOT cut these)

Procedure:

Part 1: Start this part first since the complete dissolution of the zinc will require at least an hour. Obtain four quarter- sections of a penny from the Stockroom. Examine one of them, and note the silver-colored zinc core and the outer copper layer.

The analysis will be performed in duplicate. Each sample will consist of two quarter - sections of a penny. Obtain the combined mass of two quarter-sections to 0.001 g. Record the mass, and place them into a labeled 100 ml beaker. Repeat the procedure with the other two quarter-sections.

Place the beakers in the hood, and to each one add about 20 mL of concentrated (12 M) hydrochloric acid. When gas bubbles are no longer visible, the zinc is completely dissolved and you may then discard the acid in the sink. Wash the remaining copper shells with deionized water, followed by a rinse with acetone. When the copper shells appear dry, weigh and record their mass.

Calculate the percent copper in each of the two samples, and the average value. Show your calculations clearly.

Part 2: To each of two beakers in the hood, add a small piece of copper. To one add about 10 mL of 6 M hydrochloric acid, and to the other add about 10 mL of 6 M nitric acid. What do you observe?

To another beaker in the hood, add a small piece of zinc, followed by about 10 mL of 6 M hydrochloric acid. What do you observe?

Part 3: Obtain a sample of 10 pennies from the Stockroom. Weigh each penny carefully to 0.001 g and record its mass and date.

Conclusion:

Part 1:

1. What value did you obtain for the average percentage of copper in a penny?

2. What are some possible sources of error in this analysis?

Part 2:

1. Write the balanced chemical equations for the reactions which occur when:

a) Cu metal is placed in HCl(aq).

b) Cu metal is placed in HNO₃(aq).

c) Zn metal is placed in HCl(aq).

2. What is the brown gas produced in the reaction of copper with nitric acid?

Part 3:

Prepare a graph of mass versus date for your sample of 10 pennies. Does the mass vary with date?

 m perona mike@chem.csustan.edu