Letter pubs.acs.org/jchemeduc
A(nother) Modification of the Ammonia Fountain Demonstration Ben Ruekberg* and David L. Freeman Chemistry Department, University of Rhode Island, Kingston, Rhode Island 02881, United States S Supporting Information *
ABSTRACT: A modification of the ammonia fountain demonstration is presented which does not use dyes. The procedure illustrates both the solubility of ammonia as well as the formation of coordination complexes and drives the reaction toward completion by removing a product. KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry
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www.youtube.com/watch?v=GZWc6t9Lanw. Additionally, our procedure is outlined in the Supporting Information. This demonstration was well-received by general chemistry students.
he ammonia fountain has long been a well-liked lecture demonstration. A popular modification is the incorporation of phenolphthalein and/or other indicators to give the fountain the added interest of a color change to the fountain. There are too many of these for more than a representative sample.1−4 These modifications take advantage of ammonia’s acting as a Brønsted−Lowry base. Our modification takes advantage of ammonia’s acting as a Lewis base, which allows it to form coordination compounds, perhaps most dramatically represented by the striking blue of [Cu(NH3)4]2+. For instructors using texts such as Chang and Goldsby,5 which discuss complex ion formation at the end of the chapter on acids and bases, the use of ammonia complexation of copper ions in an ammonia fountain can be an effective complement to the material in such chapters. If a 1 L flask is used for the fountain, it will contain 1/24.465 mol of ammonia at 25 °C and 1 atm pressure, approximately 0.04 M. This suggests that the use of a 0.01 M copper sulfate solution, about 2.5 g of copper sulfate pentahydrate per liter, would give the desired blue color.
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HAZARDS Various versions of the ammonia fountain have been published, and the precautions particular to the one chosen should be followed scrupulously. An implosion hazard has been reported.8 Because a partial vacuum is created in this demonstration, cracked or starred glassware must be avoided. The demonstration should be performed in a hood, and lab coat, goggles, and gloves should be worn by the presenter. Copper sulfate is irritating to skin and eyes, and toxic if swallowed. It is very toxic to aquatic life. Ammonia causes severe burns to skin and eyes. It is toxic if inhaled and very toxic to aquatic life. It is corrosive and flammable.
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The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00295. Brief description of the method used for this demonstration (PDF, DOCX)
Cu 2 + + 4NH3 ⇄ [Cu(NH3)4 ]2 +
If Kf = 1.1 × 1013,6 almost 97% of the ammonia would be converted to tetraaminecopper(II). This could make the fountain demonstration more dramatic for another reason. While water will absorb approximately 600 times its volume in gaseous ammonia (based on a 31% solubility at 25 °C)7at room temperature and 1 atm pressure, the copper ion in solution will pull the ammonia molecules out of the water allowing the water to reduce the pressure in the flask marginally faster. The ammonia-filled flask, with a tube (the exclusive opening to the flask) going into the copper sulfate solution, is suspended above the solution. A small amount of water is introduced into the flask. When the ammonia dissolves in the water and copper solution, the pressure inside the flask is reduced, and the pressure on the surface of the solution pushes it into the flask. For those unfamiliar with the ammonia fountain demonstration, we recommend references 1−3 and other articles on the subject in the Journal of Chemical Education. The setup and operation of ammonia fountains can be found on YouTube, such as https://www.youtube.com/watch?v=3CafICEol0A, https://www.youtube.com/watch?v=-z4liRirdv0, and https:// © XXXX American Chemical Society and Division of Chemical Education, Inc.
ASSOCIATED CONTENT
S Supporting Information *
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ben Ruekberg: 0000-0003-1456-7860 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Steadman, N. Ammonia fountain improvements. J. Chem. Educ. 1992, 69 (9), 764. Received: May 1, 2017 Revised: July 13, 2017
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DOI: 10.1021/acs.jchemed.7b00295 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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(2) Nicholas, N. C.; Faulk, S.; Sullivan, R. A Hand-Held Ammonia Fountain. J. Chem. Educ. 2008, 85 (8), 1063. (3) Proksa, M. Ammonia Fountain and Density Gradient Column. J. Chem. Educ. 1995, 72 (10), 931−932. (4) Griffoul, D. A. A colored ammonia fountain. J. Chem. Educ. 1931, 8 (6), 1059. (5) Chang, R.; Goldsby, K. A. General Chemistry: The Essential Concepts, 7th ed.; McGraw Hill: New York, 2014. (6) Petrucci, R. H.; Herring, F. G.; Madura, J. D.; Bissonnette, C. General Chemistry: Principles and Modern Applications, 11th ed.; Pearson: Toronto, 2016; p 847. (7) The Merck Index, 11th ed.; Budavari, S., Ed.; Merck & Co., Inc., 1989; p 81. (8) Kauffman, G. B. The ammonia fountain-an implosion danger. J. Chem. Educ. 1982, 59 (1), 80.
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DOI: 10.1021/acs.jchemed.7b00295 J. Chem. Educ. XXXX, XXX, XXX−XXX
