In the Classroom edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
The Ammonia Smoke Fountain: An Interesting Thermodynamic Adventure submitted by:
M. Dale Alexander* Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM 88003
checked by:
Daniel T. Haworth Department of Chemistry, Marquette University, Milwaukee, WI 53201
The ammonia fountain is one of the oldest and most popular chemical demonstrations. Descriptions of the ammonia fountain have appeared in high school and college chemistry texts from before the turn of the century to the present time (1–7), and chemical demonstration sources invariably contain descriptions of ammonia fountain demonstrations (8) . Figure 1 shows a diagram of a typical ammonia fountain apparatus. When a small amount of water is introduced into the flask using the dropping pipet, a portion of the ammonia gas dissolves in the water, causing a decrease in pressure. Atmospheric pressure forces more water into the flask through the other tube—dissolving more ammonia, resulting in a greater pressure decrease, and the water jets into the flask until essentially all of the ammonia has dissolved. The phenolphthalein indicator turns red as the water enters the flask. Recently, a novel variation of the ammonia fountain has been devised to illustrate chemiluminescence (9). In this demonstration, two solutions containing the reactants for a chemiluminescence reaction are drawn into the ammonia flask, and chemiluminescence occurs within the flask. Herein is described a new demonstration, which utilizes an apparatus similar to the ammonia fountain apparatus,
substituting hydrogen chloride gas for the water, as shown in Figure 2. When water is introduced into the flask, the pressure decreases as in the ammonia fountain. This results in hydrogen chloride being drawn into the flask instead of water, and a familiar reaction occurs, the combination of ammonia and hydrogen chloride to form ammonium chloride smoke (eq 1). NH3(g) + HCl(g) → NH4Cl(s)
Whereas the pressure drop in the ammonia fountain is due to the great solubility of ammonia in water, the pressure drop in the smoke fountain is due to the conversion of ammonia gas to solid ammonium chloride by reaction with hydrogen chloride. This drop in pressure results in the introduction of more hydrogen chloride gas into the reaction flask. In the case of the ammonia fountain the flow rate of the water jet is essentially constant. The flow rate of hydrogen chloride gas into the flask in the smoke fountain is not constant but periodic; that is, the smoke puffs from the end of the tube. This unexpected behavior elicits an interesting thermodynamic explanation. Experimental Section
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NH3
NH3
dropping pipet
The Apparatus A one-liter round-bottom flask is employed together with an appropriate two-holed rubber stopper and glass tubing (Fig. 2). Note that the glass tube through which the hydrogen chloride is introduced into the flask is not drawn into a jet, as is done in the ammonia fountain. The hydrogen chloride is contained in a 500-mL Erlenmeyer flask also equipped with an appropriate two-holed rubber stopper and glass tubing as indicated (Fig. 2).
glass tube dropping pipet
water and phenolphthalein
HCl
Figure 1. Ammonia fountain apparatus.
210
(1)
Figure 2. Ammonia smoke fountain apparatus.
Preparation The tubing between the two flasks is disconnected, and the dropping pipet is removed from the rubber stopper in the ammonia flask. The other tube is connected to an ammonia lecture bottle equipped with a needle valve. The flask is slowly filled with ammonia by the displacement of air as a consequence of the relatively low density
Journal of Chemical Education • Vol. 76 No. 2 February 1999 • JChemEd.chem.wisc.edu
In the Classroom
of ammonia. Moistened red litmus paper held at the open hole of the rubber stopper turns blue when the flask is filled. A hydrogen chloride gas lecture bottle equipped with a needle valve is connected to the longer tube of the other flask and the gas is slowly introduced, resulting in the displacement of air because of HCl’s greater density. Moistened blue litmus paper held at the end of the other tube turns red when the flask is filled. If lecture bottles of the gases are not available, the ammonia may be readily generated by reaction of ammonium chloride with calcium hydroxide (10); and hydrogen chloride may be generated by reaction of sodium chloride with concentrated sulfuric acid (11). Presentation Connect the two flasks with rubber tubing as indicated (Fig. 2). Fill the dropping pipet (capacity approximately 1 mL) with water and introduce it into the open hole of the rubber stopper. Slowly squeeze the bulb of the dropping pipet to introduce a small amount of water (approximately 1 mL) into the flask. Release the bulb slowly. After a few seconds hydrogen chloride gas begins to enter the flask, and the puffing effect described earlier commences. The process continues as the flask fills with smoke. Waves of ammonium chloride smoke move on the inner surface of the flask as a result of the puffing effect. If water is introduced more rapidly by squeezing the bulb abruptly so as to shoot a jet of water into the flask, the flow of hydrogen chloride gas will be considerably faster and the flask will fill with smoke more rapidly, obscuring the puffing effect.
Safety Considerations Unlike the traditional ammonia fountain, for which implosion possibilities exist (12, 13), the ammonia smoke fountain demonstration is quite safe. Precautions should be taken, however, when filling the flasks with the two gases, since both hydrogen chloride and ammonia are toxic.
the heat liberated in the reaction with 1 mL of HCl were absorbed by the 999 mL of unreacted NH 3, the temperature would increase by 6 K to 305 K with a corresponding increase of pressure from 759 torr to 774 torr. Thus these calculations indicate that if, on the one hand, the reaction is carried out under isothermal conditions such that all of the heat produced in the reaction is transferred to the surroundings (the flask, glass tubes, stopper, water, and surrounding air) the pressure in the system will decrease and more hydrogen chloride will be drawn into the flask to continue the reaction. If, on the other hand, the reaction is carried out under adiabatic conditions such that all the heat produced in the reaction is confined to the unreacted ammonia, then a rather large pressure increase will occur and no more hydrogen chloride will be drawn into the system. However, the actual reaction is not carried out under true isothermal conditions or under adiabatic conditions. When the HCl enters the flask the reaction occurs very rapidly, so that during the short reaction time the reaction zone is essentially adiabatic. A rapid burst of heat occurs in this region, accompanied by a sudden large increase of temperature and a sudden increase in pressure. The sudden increase in pressure causes a momentary interruption of the flow of HCl. The hot gas smoke mixture in the reaction zone quickly expands and rises away from the reaction zone; heat is transferred to the surroundings; more HCl is drawn into the flask; and the process is repeated. Conclusion The ammonia smoke fountain demonstration is easy to perform, is interesting to observe, and provides a means of demonstrating several important properties and concepts. Among these are the great solubility of ammonia in water, the basic nature of ammonia, the acidic nature of hydrogen chloride, and the fact that ionic compounds are solids. Concepts include the gas laws, stoichiometry, heat capacity, and thermochemistry. Literature Cited
Discussion Thermodynamics provides an explanation for the unexpected puffing effect. Consider an example in which the volume of ammonia in the flask is 1000 mL, the temperature is 298 K, and pressure is 760 torr. Suppose 1.0 mL of hydrogen chloride gas is injected into the flask. The reaction with ammonia will remove 1.0 mL of ammonia gas, and if removal of the ammonia gas were the only consequence of the reaction the pressure would decrease to 999/1000 of the original pressure, or 759 torr. But the reaction is very exothermic, and the heat produced must be accounted for. The ∆Hrx°n for the formation of NH4Cl(s) in eq 1 is calculated to be ᎑176.2 kJ mol᎑1 using ∆Hf° for NH3(g), HCl(g), and NH4Cl(s) as ᎑45.9, ᎑92.3, ᎑314.4 kJ mol ᎑1, respectively (14). Thus, 1 mL of HCl(g) (4.1 × 10᎑5 mol) on reaction with 1 mL of NH3(g) releases 7.2 J of heat (∆H °= ᎑7.2 JmL ᎑1). Using the molar heat capacity of NH3 at constant pressure (C p) of 35.1 Jmol᎑1K᎑1 (15) and assuming ideal gas behavior, the molar heat capacity of NH3 at constant volume, Cv, is calculated to be 30.9 J mol᎑1 K᎑1. This gives for 1000 mL of NH3 gas at 298 K, 760 torr, a heat capacity of 1.3 J. Consequently, if all
1. Venable, F. P.; Howe, J. L. Inorganic Chemistry; The Chemical Publishing Co.: Easton, PA, 1898; p 124. 2. Holmes. H. N. General Chemistry; Macmillan: New York, 1923; p 227. 3. Brownlee, R. B.; Fuller, R. W.; Hancock, W. J; Sohon, M. D. First Principles of Chemistry; Allyn and Bacon: New York, 1937, p 328. 4. Foster, W.; Alyea, H. N. Introduction to Chemistry; Van Nostrand: New York, 1947, p 549. 5. Umland, J. B. General Chemistry; West: Minneapolis, 1993; p 954. 6. Bodner, G. M.; Pardue, H. L. Chemistry, An Experimental Science; Wiley: New York, 1995; p 601. 7. Chang, R. Essential Chemistry; McGraw-Hill: New York, 1996; p 350. 8. Shakhashiri, B. Z. Chemical Demonstrations, Vol. 2; The University of Wisconsin Press: Madison, 1985, p 205. 9. Thomas, N. C. J. Chem. Educ. 1990, 67, 339. 10. Partington, J. R. Textbook of Inorganic Chemistry; Macmillan: London, 1950; p 514 . 11. Partington, J. R. Op. cit., p. 209. 12. Weaver, E. C. J. Chem. Educ. 1944, 21, 199. 13. Kauffman, G. B. J. Chem. Educ. 1982, 59, 80. 14. CRC Handbook of Chemistry and Physics, 75th ed.; Lide, D. R., Ed.; CRC: Boca Raton, FL, 1994; p 5-9.16. 15. Ibid.; p 6-141.
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