I have always been fascinated by chemical reactions that form shiny, metallic mirrors. The most popular version of this experiment is perhaps the reduction of silver ions to silver metal via the Tollen’s test.1-2 In addition to the formation of silver mirrors, the formation of copper mirrors have also been reported.3-5 A very simple method of producing a copper mirror involves heating copper (II) formate in a test tube. In this reaction, the copper (II) formate undergoes thermal decomposition:3
Cu(CHO2)2 (s) → Cu(s) + CO2(g) + CO(g) + H2O Equation 1
When performing this reaction, I often observe aerosolized particles of copper metal and other colored fumes escaping the test tube (Video 1). Therefore, this reaction should be performed in the fume hood.
Notice that during the reaction, formate ion is oxidized to CO2 and CO, while the Cu2+ ion is reduced to metallic copper (Equation 1). Thus, this reaction can be described as a redox reaction in which a singular reactant is simultaneously oxidized and reduced.
I began to wonder what other ionic compounds could be thermally decomposed to form a metal mirror. I hypothesized that the best candidates would be ionic compounds comprised of an easily reduced metal (like copper or silver) paired with an easily oxidized anion. I reasoned that if formate served as an easily oxidized anion then its two-carbon congener, acetate, might work as well. Thus, I decided to see if copper (II) acetate or silver acetate could be reduced by thermal decomposition into their respective metal mirrors. You can see the results of my investigations below (Video 1).
Video 1: Simple formation of metal mirrors, Tommy Technetium's YouTube Channel, May 9, 2021
Sure enough, both copper (II) acetate and silver acetate formed metal mirrors! After some literature searching, I noticed that people have studied thermal decomposition of both copper (II) acetate and silver acetate, as well as other metal acetates.6-9 Sure enough, people have observed that such reactions produce copper and silver in their metallic forms. There are a host of side reactions that occur during the thermal decomposition of both copper (II) acetate and silver acetate. Nevertheless, the formation of metallic copper and silver have been described by the following reactions:7,9
2 Cu(C2H3O2)2(s) + 7 O2(g) → 2 Cu(s) + 8 CO2 + 6 H2O Equation 2
4 AgC2H3O2(s) + 7 O2(g) → 4 Ag(s) + 8 CO2 + 6 H2O Equation 3
What other metal salts might form mirrors via thermal decomposition? If you try some of these experiments on your own – be sure to use the fume hood – but also let me know what you discover.
I’d love to get my hands on some platinum formate or gold acetate…
Happy Experimenting!
References
- Shakhashiri, B.Z., >Chemical Demonstrations: A Handbook for Teachers of ChemistryVolume 4, pp. 240-243.
- Kuntzleman, T., Holiday Reflections on the Silver Mirror Demonstration, Dec 2020.
- Pike, R. D. Metal in Metal Salts: A Copper Mirror Demonstration. J. Chem. Educ. 2010, 87, 1062-1063.
- Nikoloska, M.; Petruševski, V. An Improved Copper Mirror Demonstration. J. Chem. Educ. 2011, 88, 1406.
- Hill. J. W.; Foss, D. L.; Scott, L. W. A Copper Mirror: Electroless Plating of Copper. J. Chem. Educ. 1979, 56, 752.
- Keller, A.; Korosy, F. Volatile Cuprous and Silver Salts of Fatty Acids. Nature, 1948, 162, 580-582.
- Nakano, M.; Fujiwara, T.; Koga, N. Thermal Decomposition of Silver Acetate: Physico-Geometrical Kinetic Features and Formation of Silver Nanoparticles. J. P. Chem. C. 2016, 120, 8841-8854.
- Judd, M. D.; Plunkett, B. A.; Pope, M. I. The Thermal Decomposition of Calcium, Sodium, Silver, and Copper (II) Acetates. J. Thermal Anal. 1974, 6, 555-563.
- Lin, Z.; Han, D.; Li, S.; Study on Thermal Decomposition of copper (II) acetate monohydrate in air. J. Therm. Anal. Calorim. 2012, 107, 471-475.
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
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.
Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models.
Planning and carrying out investigations in 9-12 builds on K-8 experiences and progresses to include investigations that provide evidence for and test conceptual, mathematical, physical, and empirical models. Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
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.