In-Class Demonstration

In-Class Demonstration

Pennies That Shine: A Chemical Change

Student Introduction

While the concept of oxidation and reduction reactions may initially seem confusing, these processes are quite common. Everyone has seen a rusty metal bridge or a green-colored copper roof. These are everyday examples of oxidation and reduction reactions. The iron in the steel of the bridge oxidizes over time to produce a compound known as iron (III) oxide, or rust, and the copper roof oxidizes and reacts with sulfur dioxide and carbon dioxide in the air to produce a characteristic green “patina” color. These reactions are examples of chemical changes, processes in which one or more different materials are produced from one or more initial compounds.

In this demonstration, we will use common household materials to remove the copper oxide from tarnished pennies. The reactions taking place are given below.

HC2H3O2 + NaCl → HCl + NaC2H3O2

vinegar + salt → hydrochloric + sodium

acid acetate

CuO + HCl → CuCl2 + H2O

copper (II) oxide + hydrochloric → copper (II) chloride + water

acid

Carefully observe as your instructor performs this experiment and record important findings and observations in your notebook.

Questions

1.  Why is this reaction an example of a chemical change?

2.  Balance the equations used in this demonstration.

Instructor’s Resource Guide

Materials required:

5 –10 dull pennies

¼ cup white vinegar (dilute acetic acid)

2 teaspoons salt (NaCl)

1 250-ml glass beaker or plastic dish

1 bottle water

Tweezers

Paper towels

Procedure:

1.  Place the pennies in the beaker or plastic dish.

2.  Add the white vinegar to the beaker.

3.  Add 2 teaspoons of salt to the vinegar and stir to dissolve.

4.  Swirl the pennies in the solution until shiny.

5.  Using the tweezers, remove the pennies, wash with the water to remove the solution and place on the paper towel to dry.

6.  Carefully dispose of the residual solution.

Time required: 10 minutes

Safety: Instructors should assess the risks associated with the chemicals used in this demonstration. Protective clothing and eye goggles should be used. Dispose of chemicals properly.

Reference: Rosenhein, Laurence D. J. Chem. Educ., 78:513 (2001).

Answers to Questions

1.  These reactions represent chemical changes because the reactants or starting materials are converted into different compounds. In the first reaction, acetic acid (vinegar) reacts with sodium chloride to produce hydrochloric acid and sodium acetate. In the second reaction, the copper (II) oxide on the tarnished pennies reacts with hydrochloric acid to produce copper (II) chloride and water.

2.  HC2H3O2 + NaCl → HCl + NaC2H3O2 (The equation is balanced as provided.)

CuO + 2 HCl → CuCl2 + H2O

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