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Chemical Education Today
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Letters
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Reaction to Why Do We Teach Equilibrium Calculations?
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Stephen J. Hawkes
Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003
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September 2004
Vol. 81 No. 9
p. 1265
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| Full Text |
The author replies to Lewis.
Computer programs assist in understanding equilibrium because they can deal with the simultaneous solution of multiple equilibria. Solubility of barium sulfate in 0.1 M sodium sulfate is affected by solubility of the ion pair and by the activity coefficient so that the usual answer of solubility product/concentration is in error by a factor of 32. Among all the solubilities in 0.1 M negative ion like this, about a third are in error by more than 100. In pure water, a third are in error by more than 10 (1). Calculation of the pH of buffers is often in error by up to 0.3 unit (2). It is not pointed out to beginners that pH = {–log[H+]} – (0.2 ± 0.2). Multiple equilibria and the effect of ionic strength are neglected in beginning chemistry to simplify the math, deceiving students into believing that they have mastered a skill when they only produce wrong answers that match a test key. This is illustrated by three texts (3) that calculate the solubility of calcium carbonate as an example, neglecting the hydrolysis of the carbonate ion.
Don Lewis does not accept that equilibrium is a rare phenomenon other than in chemists' laboratories. The entire ecosystem is in disequilibrium as is the earth's crust, the oceans, the atmosphere, the sun, and the stars. Ruins of ancient monuments illustrate their disequilibrium with the environment. It is naive to take issue with John Moore's statement (4) that one major cause of ineffective teaching is the assumption that "Students who can answer numeric questions on exams actually understand the principles of chemisttry." I had to learn this error the hard way. Lambeth and Robinson (5) showed that the concept of equilibrium is often not understood by students who can plug numbers successfully into mass action expressions.
It is true that chemistry involves questions that are answered with numbers. That does not mean, however, that each and every aspect of chemistry must be taught quantitatively or with quantitative rigor. The chapters of equilibrium calculations that encumber our texts are out of all proportion to their usefulness. Most students will never use them, and their emphasis obscures the understanding of the nature and cause of equilibrium.
"Rigor" in introductory chemistry is often equated with quantitation. Consequently the understanding of chemical reactions and properties is obscured. This was illustrated by Stumpo (6) who asked students to calculate ΔE of a reaction, and then on another question on the same test asked a question aimed at its meaning. 77% of the students calculated correctly, but only 24% showed understanding of its meaning.
The ability to calculate a number does not measure understanding of the number.
Literature Cited
- Hawkes, S. J. CHEM13 News 1996, 1.
- Hawkes, S. J. Chem. Educator 1997, 1 (6): S1430-4171.
- Hawkes, S. J. J. Chem. Educ. 2000, 77, 321–326.
- Moore, J. W. J. Chem. Educ. 2004, 81, 7.
- Lambeth, J. M.; Robinson, W. R. Presented at the 210th ACS Conference, Division of Chemical Education Session, Chicago, Aug 1995; CHED Newslett. 1995, Fall; Abstract 336.
- Stumpo, V. M. J. Chem. Educ. 1992, 69, 459–462.
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| More Information |
 Citation |
Hawkes, Stephen J. J. Chem. Educ. 2004 81 1265.
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Keywords |
Equilibrium; General Chemistry; Teaching / Learning Aids
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History |
Created:
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July 28, 2004
August 10, 2004
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