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Chemical Education Today
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Letters
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The Reaction Quotent Is Unnecessary To Solve Equilibrium Problems. The Limitation of a Qualitative Reasoning
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Michiel Vogelezang
Radboud University Nijmegen, Institute for Teacher and School, Nijmegen, The Netherlands
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March 2006
Vol. 83 No. 3
p. 383
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| Full Text |
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In this Journal there has been a discussion about the necessity of the reaction quotient for solving equilibrium problems (1–4). Especially, the contribution of Robert Lederer (3) caught my attention, and I tried to do his experiment with 0.2 M acetic acid. Having created a situation like the one described by Lederer, namely a bulb that did not light up when the electrodes were immersed in the solution, I did not notice an increase of the conductivity when diluting the solution to 0.1 M as the bulb still did not burn. This was what I expected, because there is a shift of equilibrium to the side of the ionized products, but the concentration (or activities) after dilution will be smaller than at the start of the experiment, although it will not be twice as small. I agree with Lederer that the total amount of ions in the solution will be greater than before diluting, but the actual conductance of the liquid is the result of the concentrations (or activities) and not of their total amount. This point is not recognized by Paul Matsumoto in his reaction (4). I fully agree with him that the dissociation of the acetic acid will be complete in an endless dilution, but this does not mean that this endless diluted solution, which, in fact, will be water, will be a good conductor.
In this discussion we can have a look at the conductance measurements of MacInnes
and Shedlovski (5), whose results are also listed
in Robinson and Stokes (6). In the box below, some
of their values of the equivalent conductance at various molarities of the
acetic acid are listed. These values show how the actual concentrations drop
off, but slower than the initial acetic acid molarity. Although MacInnes and
Shedlovski did not report on a concentration of 1.0 M, we can assume that the
shift in the values listed also will hold good for this concentration. The
reasoning given by Robert Lederer is a good qualitative one, but in this case
a more precise look would be necessary.
Literature Cited
- Matsumoto, P. S. J. Chem. Educ. 2005, 82, 406–407.
- Silverstein, T. P. J. Chem. Educ. 2005, 82, 1149.
- Lederer, R. J. Chem. Educ. 2005, 82, 1149.
- Matsumoto, P. S. J. Chem. Educ. 2005, 82, 1150.
- McInnes, D. A.; Shedlovski, T. J. Am. Chem. Soc.
1932, 54, 1429–1438.
- Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworths:
London, 1959.
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| More Information |
 Citation |
Vogelezang, Michiel. J. Chem. Educ. 2006 83 383.
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Keywords |
Equilibrium; First-Year Undergraduate / General; High School / Introductory Chemistry; Mathematics / Symbolic Mathematics; Physical Chemistry; Textbooks / Reference Books
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History |
Created:
Last Updated: |
1/31/2006
2/6/2006
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