One of my favorite chemistry demonstrations is the “Copper to Silver to Gold” experiment1 (VIDEO 1), in which a penny is first plated with zinc and then heated to form brass, an alloy of copper and zinc. The heat causes zinc to diffuse into the copper in the penny. The zinc plate imparts a beautiful silver color to the penny. The brass that is formed upon heating has a stunning golden color.
Video 1: Copper to "Silver" to "Gold", Tommy Technicium YouTube Channel (accessed 2/1/2021)
My students and I have performed this experiment many times, and students often keep the golden colored pennies as keepsakes. Unfortunately, the zinc plated pennies lose their silver color over a few days, presumably because zinc diffuses into the copper even at room temperature.2 A new experiment published in the Journal of Chemical Education describes how copper can be coated with tin.2 In this case, the silver-colored coating imparted by the tin appears to last for quite some time – perhaps indefinitely! I tried to see if this tin-plating experiment would work on pennies (VIDEO 2).
Video 2: Plating pennies with tin, Tommy Technicium YouTube Channel (accessed 2/1/2021)
The pennies I plated with tin using this method have kept their silver color for weeks and show no sign of discoloring! The authors claim that the stability of the tin-plated copper as opposed to the zinc-plated copper can be understood by comparing the enthalpies of sublimation of tin (301 kJ mol-1) and zinc (130 kJ mol-1).2 Based on these values, the bonds between tin atoms are stronger than those between zinc atoms. As a result, zinc is more likely to diffuse into the copper than tin – and it does so even at room temperature. When zinc diffuses into the copper, it loses its silvery coat because brass is formed.
A few other features of the experiment are of note. The original “Copper to Silver to Gold” experiment requires the use of caustic sodium hydroxide solution, while the reagents required for this new tin-plating experiment are quite benign by comparison. Also, the tin-plated pennies can be heated to form bronze, which is a golden-colored alloy of copper and tin. I tried to form bronze on tin-plated pennies with mixed success. I could achieve a golden color by heating tin-plated pennies, but I had trouble getting a uniformly golden color. In my opinion, if you wish to plate pennies with a golden color, the formation of brass via the original “Copper to Silver to Gold” experiment is superior to this new experiment. However, if you wish to give pennies a nice silvery coat, this tin-plating experiment is the way to go!
Given that copper is higher up in the electrochemical series than tin, it might seem curious that the tin plates out on copper in this experiment. In fact, it is not the case that an electrochemical reaction between tin ions and copper metal occurs in this experiment. Rather, tin metal in granular tin is oxidized to Sn2+(aq):
Sn(s) → Sn2+(aq) + 2 e-
The electrons transfer into the surface of the copper, where dissolved tin ions are then reduced to tin metal. This forms the tin plate:
Sn2+(aq) + 2 e- → Sn(s)
The overall process involves granular tin being deposited on the surface of the copper. Figure 1 below summarizes this whole process. The authors state that this happens because the tin atoms are more stable when interacting with copper atoms than they are when interacting with each other.2
Figure 1: Principle of Sn Plating - Reprinted with permission from Suzuki, T.; Inoue, M., *Cu Plating with Sn and Subsequent Bronze Formation under Mild Conditions, J. Chem. Educ. Copyright 2021 American Chemical Society.
I will be interested to see how the pennies that I plated with tin retain their silver color – or not – over the next few months. I’m hoping that the silver color lasts for years. If you try out this experiment, let me know how the silver coat works for you. Also, if you get the bronze coating to form with a nice, uniform golden color, be sure to send me a few tips on how to do so.
Happy experimenting!
References
- Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin: Madison, WI, 1992; Vol. 4, pp 263−268.
- Suzuki, T.; Inoue, M., *Cu Plating with Sn and Subsequent Bronze Formation under Mild Conditions, J. Chem. Educ. 2021, DOI: 10.1021/acs.jchemed.0c00552.
*See Accessing Cited Articles.
Safety
Safety: Video Demonstration
Safety: Video Demonstration
Demonstration videos presented here are not meant as tools to teach chemical demonstration techniques. They are meant as a tool for classroom use. The demonstrations may present safety hazards or show phenomena that are difficult for an entire class to observe in a live demonstration.
Those performing the demonstrations shown in this video have been trained and adhere to best safety practices.
Anyone thinking about performing a chemistry demonstration should first read and then adhere to the ACS Safety Guidelines for Chemical Demonstrations (2016) These guidelines are also available at ChemEd X.
General Safety
General Safety
For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies
NGSS
Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas.
*More information about this category of NGSS can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions.
"Matter and its Interactions help students formulate an answer to the question, “How can one explain the structure, properties, and interactions of matter?” The PS1 Disciplinary Core Idea from the NRC Framework is broken down into three subideas: the structure and properties of matter, chemical reactions, and nuclear processes. Students are expected to develop understanding of the substructure of atoms and to provide more mechanistic explanations of the properties of substances. Chemical reactions, including rates of reactions and energy changes, can be understood by students at this level in terms of the collisions of molecules and the rearrangements of atoms. Students are able to use the periodic table as a tool to explain and predict the properties of elements. Using this expanded knowledge of chemical reactions, students are able to explain important biological and geophysical phenomena. Phenomena involving nuclei are also important to understand, as they explain the formation and abundance of the elements, radioactivity, the release of energy from the sun and other stars, and the generation of nuclear power. Students are also able to apply an understanding of the process of optimization in engineering design to chemical reaction systems. The crosscutting concepts of patterns, energy and matter, and stability and change are called out as organizing concepts for these disciplinary core ideas. In the PS1 performance expectations, students are expected to demonstrate proficiency in developing and using models, planning and conducting investigations, using mathematical thinking, and constructing explanations and designing solutions; and to use these practices to demonstrate understanding of the core ideas."
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
*More information about all DCI for HS-PS1 can be found at https://www.nextgenscience.org/dci-arrangement/hs-ps1-matter-and-its-interactions and further resources at https://www.nextgenscience.org.
Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Assessment is limited to chemical reactions involving main group elements and combustion reactions.
Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.