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

1. Shakhashiri, B.Z., >Chemical Demonstrations: A Handbook for Teachers of ChemistryVolume 4, pp. 240-243.
2. Kuntzleman, T., Holiday Reflections on the Silver Mirror Demonstration, Dec 2020. 
3. Pike, R. D. Metal in Metal Salts: A Copper Mirror Demonstration. J. Chem. Educ. 2010, 87, 1062-1063.
4. Nikoloska, M.; Petruševski, V. An Improved Copper Mirror Demonstration. J. Chem. Educ. 2011, 88, 1406.
5. Hill. J. W.; Foss, D. L.; Scott, L. W. A Copper Mirror: Electroless Plating of Copper. J. Chem. Educ. 1979, 56, 752.
6. Keller, A.; Korosy, F. Volatile Cuprous and Silver Salts of Fatty Acids. Nature, 1948, 162, 580-582.
7. 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.
8. 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.
9. 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.