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The school day before I read Paul S. Matsumoto’s article (1), I performed a demonstration for my AP chemistry students that they found interesting. I immersed the electrodes of a light-bulb conductivity apparatus into 1.0 M acetic acid. No observable conductivity. I then proceeded to dilute the solution with distilled water, doubling the solution volume. The bulb lit up. My students were perplexed; the observation was counterintuitive to what they expected. After writing the dissociation of the weak acid, and the equilibrium expression, the students plainly see that cutting all the equilibrium concentrations by half will disturb the equilibrium, resulting in a shift to produce more ions in solution. The students realize this because the new reaction quotient, Q, will have a value less than the Ka; the reaction shifts right. Without an understanding of Q—what it really means —this problem cannot be adequately solved. Nor can precipitation problems, where Q and Ksp will determine whether precipitation occurs in a solution where various ion concentrations are mixed. Mr. Matsumoto’s students are to be congratulated for discerning an interesting mathematical procedure. Exclusively utilizing this algorithm, however, short-cuts the understanding of the chemistry involved. Students of chemistry should be challenged to understand why something occurs, and not to be satisfied with how to perform the often mundane calculations. The title of the article is unfortunate and misleading. The reaction quotient is necessary to solve many equilibrium problems. The meaning of Q should always be taught, and the calculation of it utilized, to encourage proper appreciation of chemical equilibria. Literature Cited- Matsumoto, P. S. J. Chem. Educ. 2005, 82, 406–407.
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