Chemical Education Today
Letters Elasticity to Measure Thermodynamic Properties The elegant experiment “Rubber Elasticity: A Simple Method for Measurement of Thermodynamic Properties” by John P. Byrne (J. Chem. Educ. 1994, 71, 531) can be made even simpler if one uses an inexpensive toploading electronic balance to measure the tension force of the rubber band, instead of the triple-beam balance Byrne employed, and an old coat hanger to fashion a wire harness and hook to stretch the rubber band. I have repeated Byrne’s experiment using a 200-g capacity (±0.01 g accuracy) electronic balance (Acculab V-200) supported on a ring stand and ring, above a water bath consisting of a 0.5-L tall-form beaker of water heated by a 550-W stirrer-hotplate. A schematic of the apparatus is shown below. electronic balance
wire harness
water bath
rubber band
stirrer-hotplate
The rubber band, prepared as described by Byrne, was stretched between the wire harness supported by the pan of the balance and an adjustable hook held in position by a three-finger clamp; since electronic balances use magnetic forces during weighing to maintain the pan at a fixed null position, the rubber band is held at a constant length during the experiment. The values of the thermodynamic functions, fu, the contribution of changes in internal energy with length to the restoring force, and fs, the entropic contribution to the restoring force, that I obtained are consistent with those of Byrnes: (δU/δl)T = fu = 0.40 ± 0.02 N
Principal Net Equations Betty J. Wruck’s article, “Reinforcing Net Ionic Equation Writing: Second Semester” (J. Chem. Educ. 1996, 73, 149), deals with a basic concern of general chemistry instructors. However, two important caveats must be added. First, because the expression “molecular” has been given to equations in which all species are written indiscriminately as molecules regardless of their degree of ionic or covalent character, use of the term “ionic” for this type of equation advocated by Wruck as well as by numerous textbooks and laboratory manuals is often misleading to students. Indeed, examination of some students’ efforts in this field seems to point to their apparent belief that the method involves writing everything as ionic. For this reason and because such equations are “principal”, i. e., they show both reactants and products in the principal form (covalent or ionic) in which they exist, and “net”, i. e., they omit extraneous, nonreacting species, my mentor and Doktorvater, the late John F. Baxter, Jr. (1), advocated the term “principal net reactions”. Because a large proportion of beginning students experience considerable difficulty in writing such equations, a detailed discussion of and directions for consistent writing of principal net equations, along with specific examples (2), has proved to be of value to students and instructors alike. Second, several chemists have pointed out that the principal species in aqueous ammonia is molecular NH3 (3– 6), even though Tuttle (7) reports that NH 4OH exists as a highly labile complex with an extremely short lifetime (10{10 s). However, Wruck writes it as NH4OH. Indeed, one of the reasons that John Baxter and I advocated the use of “Hydrated Cations in the General Chemistry Course” (8) was to avoid consistently the necessity of using NH4OH or NH 3 + H2O in equations for the hydrolysis of hydrated metal cations or the precipitation of gelatinous metal hydroxides by the action of aqueous ammonia on solutions containing hydrated metal cations such as Al(H 2 O) 6 3+, Cr(H 2 O) 6 3+, Mg(H2 O)62+ , and Zn(H2O)4 2+. Furthermore, the Brønsted– Lowry concept (9) permits hydrolysis to be viewed as a normal acid–base reaction. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.
and (δS/δl)T = fs = 0.78 ± 0.02 N Jonathan Mitschele Saint Joseph’s College Windham, ME 04062
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Journal of Chemical Education • Vol. 74 No. 4 April 1997
Kauffman, G. B. J. Chem. Educ. 1987, 64, 752. Kauffman, G. B. J. Coll. Sci. Teaching 1979, 9, 83. David. J. B. J. Chem. Educ. 1953, 30, 511. Laing, M. Spectrum 1988, 26(4), 11. Yoke, J. T. , J. Chem. Educ. 1989, 66, 310. Kauffman, G. B. J. Chem. Educ. 1991, 68, 534. Tuttle, Jr., T. R. J. Chem. Educ. 1991, 68, 553. Kauffman, G. B.; Baxter, Jr., J. F. J. Chem. Educ. 1981, 58, 349. Kauffman, G. B. J. Chem. Educ. 1988, 65, 28.
George B. Kauffman California State University, Fresno Fresno, CA 93740
