Below is the entire lesson plan for the lab experiment that I use with my students to perform the "Gold Penny Lab". We have a friendly classroom competition with this lab to see who can make the most convincing gold penny and the winners, as determined by student votes, are inducted into the "79 Club" (because the atomic number for gold is 79) and are dubbed "Master Alchemists". And who said chemistry couldn't be fun?
GOLD PENNY LAB
Are you a Master Alchemist? The winners of this lab will be awarded with membership into the prestigious “79 Club”, will be given a Certificate of Achievement and will have their picture hung on the Wall of Fame.
Introduction:
In this lab you will turn a copper penny into a “silver” penny and finally into a “gold” penny. But, be careful, looks can be deceiving. During the lab you will see how heated copper attracts zinc and bonds to form a Zn-Cu mixture, this is called brass. Brass is a copper-zinc alloy. An alloy is a mixture of two or more metals dissolved in each other when molten. The “silver” penny is really a coating of zinc attached to the copper shell of the penny. Heating the zinc coating causes it to migrate inward, into the copper penny, and the resulting alloy of zinc and copper reflects a gold color.
What is alchemy? Alchemy is the medieval forerunner of chemistry. Alchemists were commissioned by Kings to produce gold from other materials (unsuccessfully), and through their experiments a new science was born called Chemistry. Today, you are going to be a medieval alchemist and attempt to make the most convincing “gold” coin possible! The King’s Court will vote and the winners will be rewarded with admittance into the "79 Club"!
Pre-Lab Questions
What is the atomic number of gold? The winners of this lab inducted into the ___ Club (see a coincidence?)
What is an alloy?
Brass is typically composed of what metals?
Why must the pennies be put in sodium chloride and vinegar? (Read Procedure below)
Why should you not handle the cleaned pennies with your hands? (Read Procedure below)
Materials (link below re-direct to Amazon.com):
Zinc (granular), 1.0 grams
4 Copper pennies
1 M ZnCl2 solution, 25 ml
To make your own 1M ZnCl2 solution - Mix 68 grams ZnCl2 powder with 500 ml of distilled water)
Sodium chloride, 3.0 grams (table salt)
Vinegar, 15 ml
Equipment
Procedure:
Mix together 3.0 grams of sodium chloride and 15 ml of vinegar in a clean, 100 ml beaker.
To clean the copper pennies, place them in the sodium chloride/vinegar solution until they are shiny.
Remove the pennies and rinse them thoroughly with water. Dry with a towel. Do not handle the clean pennies with your hands. The oils from your skin can interfere with the reaction.
Mix together 1.0 grams of granular zinc, and 25 ml of 1 M zinc chloride solution in a clean evaporating dish.
Using a ring stand set up with a Bunsen burner to medium heat, carefully and gently heat the mixture to steaming. Cover with a large watch glass. Turn down the heat if the solution begins to splatter.
Using tongs, immerse two pennies in the mixture until it is completely coated with “silver”.
Use tongs to remove the penny. CAUTION: the penny will be very hot. Carefully dip the penny into a 400 ml beaker of distilled water. Shine with a towel. The penny should now appear silver. Do not touch the penny with your fingers. The oils on your hand will ruin the reactions in the following step.
Using tongs, gently heat the penny in the burner flame until the penny turns gold. Immediately dip the penny into a fresh beaker of distilled water. The penny will be extremely hot and should be handled with tongs until it has cooled for several minutes.
Clean-up:
(Follow all disposal laws for your area. Refer to MSDS forms as needed)
Any leftover NaCl and vinegar can be diluted with water and rinsed down the drain.
Place any leftover zinc and ZnCl2 solution a jar labeled "Waste" on the middle counter in the lab area.
Wash and scrub all lab equipment making sure to remove any residue
Put away all lab equipment where it belongs; wipe down station with cleaning spray and paper towel
Wash your hands before leaving the lab area
Data and Observations:
Prepare a data table that includes:
Starting mass of penny (do for two different pennies)
Mass of NaCl used
Volume of vinegar used
Mass of zinc used
Volume of ZnCl2 used
Final mass of penny (do for two different pennies)
Post-Lab Questions
Be sure to complete your data table including observations from the lab and include answers to the following questions:
What was the original appearance of the penny before you began the lab?
What happens to the pennies when they are placed in the NaCl/vinegar solution?
If the penny was changed to solid silver, what would its mass be? Show all your work. Assume the volume of a penny is 0.35 ml.
If the penny was changed to pure gold, what would its mass be? Show all your work. Assume the volume of a penny is 0.35 ml.
Based on your answer from question 4, determine how much your penny is worth if the price of gold is $1,100/oz. (Hint: you will need to convert grams to ounces using dimensional analysis! 1 g = 0.035 oz)
Based on the density of your penny, which metal is your penny mostly made of? Prove it using the densities below.
Zinc (Zn) = 7.1 g/ml Silver (Ag) = 10.5 g/ml Gold (Au) = 19.3 g/ml
Copper (Cu) = 8.9 g/ml Platinum (Pt) = 20.5 g/ml Brass (alloy) = 8.40 g/ml
Conclusion
*Your conclusion must use Chemistry terms and information including the reactions involved and the ionic bonds formed throughout this experiment. This post-lab will be graded on how well you can connect this lab with the Periodic Table Lectures and Ionic Bonding Lectures.
RSVCP format for your conclusion:
Write a paragraph in RSVCP format that describes the results of the lab.
ReState the purpose of the lab.
Verify conclusion by providing 3 or more results. This should include all numerical findings and their significance.
Provide a Counterclaim by addressing specific experimental error and suggest possible experimental improvements.
Provide importance of the experimental process by providing a specific real-world application.
