Introduction
A copper penny is heated and suspended above the surface of acetone in a beaker. The copper demonstrates heterogenous catalysis in the combustion of acetone to carbon dioxide and water. This exotherm keeps the penny glowing. In the absence of oxides on the copper surface, the copper glows yellow in the reducing gas mixture. The role of oxygen in the reaction can be shown by covering the top of the flask with a heat resistant mat: the copper will quickly cool and dim. The effluent gases have been shown by infrared spectroscopy1to mostly consist of carbon dioxide and water, with small amounts of ketene.The flicking on the surface of the penny is caused by the changing of temperature. As the vapor touches the surface, it cools the penny down and as it burns, it reheats the penny. The convection of air around the penny may also contribute to this effect.
(CH3)2CO (acetone) (Cu)> CH2CO (ketene) + CH4 (methane) (ΔH0 = 19.30 kcal)
(CH3)2CO + 4O2 —> CO2 + 3H2O (ΔH0 = -403.9 kcal)
CH2CO + O2—> H2O + CO2
CH4 + O2—> H2O + CO2
To Conduct Demonstration:
- Pour 5mm of acetone in a 250mL beaker.
- Grasp the nichrome wire 8" - 10" above the penny and hold the penny in the flame of a Bunsen burner or torch flame until it is red hot.
- Position the wire so that the penny is suspended 2 cm above the base.
- Dim the lights to view the hot glowing penny. Convection waves are visible crossing the surface of the penny. It may glow for an entire lecture period due to the heat released by the reaction.
Alternative method:
Replace coin with copper wire however thin wire may cool too quickly to maintain the reaction, or use a pre-1997 Canadian penny.
Safety
Goggles should be worn. Perform in a well-ventilated area or hood since toxic ketene gas and methane are produced. Handle the penny and wire carefully to avoid burns and so that it does not ignite the beaker of flammable acetone (flashpoint is -20 oC). Keep all flames away from the open beaker of acetone.
Reference:
1. McGarian, J. Chem. Educ., 1976, 53, 776 (DOI: 10.1021/ed053p776)
2. B.Z. Shakhashiri; Chemical Demonstrations: A Handbook for Teachers of Chemistry; Wisconsin; 1985; Volume 2; P. 216-219
3. Purdue University Department of Chemistry, Lecture Demonstration Movie sheets: http://chemed.chem.purdue.edu/demos/main_pages/19.5.html