edited by: DAVID A. PHILLIPS Wabash College Crawfordsville,IN 47933 PRUDENCE PHILLIPS CrawfordsvilleHigh Schwl Crawfordsville.IN 47933
Why Teach Solution Equilibrium?
The Goals of Teaching Solution Equilibrium
James N. Butler Division of Applied Sciences Haward Univer~iN Cambridge. MA 02138
Paul T. Ruda Cleveland Hill U.F.S.D. at CheeMowaga Mapleview Drive Cheektawaga. NY 14255
Calculations of pH in solutions of acids and bases, calculations of the soluhilitv of minerals and orecioitates. and . . calrulations of the extent of complex formation between orranic lirands and metal ions are all Dart of the traditional curricuium of inorganic and physic2 chemistry. More recently, they have been adopted by those teaching geochemistry and environmental chemistry because of their relevance to the processes taking place in the weathering of rocks, the transformations of natural waters, and the fate of pollutants in the aquatic environment. Students read in the newspapers that "the pH of rain is normally 5.6" and in their textbooks that "the pH of pure water is 7.0." Why the difference? They read that acid rain of DH4.0 leaches toxic heavy metals out of soil and rocks and in& streams and lakes. t ow does this happen? They read that lime is heing added to Adirondack lakes to counteract the acid rain. On one level this is simple neutralization of acid by base. But how much to add? How long will the treatment he expected to last? What effect will it have on the mobilized trace metals? Would these mobilized trace metals he removed from drinking water supplies by the aluminum sulfate added for coagulation before filtration? At the high school and college general chemistry level, the curriculum tends to he limited to the simple proportions of stoichiometm and to the reometric diaerams and models of molecular structures, hutihe quantitative understanding of solution eauilibria reauires some facilitv with aleebra. Nothine more than quadratic equations is required in most cases, hut the leneth and comolexitv of mani~ulationsoften discouraee anyone who is not well-practiced in these matters. What then are the most important aspects of solution equilihrium to teach a t the elementary level? I would select the concept of pH as a master variahle controlling acid-base equilibria. Students are introduced to pH as a surrogate for "acidity" or "basicity," hut an even more important aspect is often neglected or poorly understood. This is the idea that if one knows the DH of a solution. then the ratio of acid to conjugate base €0; all acidbase pairs in that solution is also known. Such a eeneral theorem is easv to Drove: it follows from the ionization equilibrium for a weak acidbase pair:
My students are involved in a course of study that follows the New York State Regents Chemistry Curriculum, which groups kinetics and equilihrium into one unit. In 1981 the Reeents Chemistm svllabus was revised to allow the classroom te&her to reorga&'and emphasize topics of his or her choice. Therefore in my class, I generally treat solution equilihrium as a transition topic. This offers many excellent opportunities t o tie toeether the other tooics that I Dresent. One G m y goals in teaching solution equilibrium is to review the eeneral c o n c e ~of t eauilihrium and to make sure that the students rememher the characteristics of an equilihrium. We approach equilihrium from the macroscopic viewpoint as a steady-state system and a dynamic-reversible system a t the microscodc level. I t is i m ~ o r t a n for t students to he able to recogniz; a steady-state sjstem hased on their observations, whether aided lw instruments or not. The conceot of a closed system is critical to their understanding of theiteady state. Phase eauilihrium is introduced earlv in the curriculum. At that time I take a large, clear plastic jar, fill i t part way with water, cover it, and leave it on the window sill. The varied conditiona that occur allow the students to observe a variety of steady states. The iar is a source of many discussions and serves a centerpoint of the review. In my unit on water and solutions, I use the concepts of solution equilihrium in two ways. The first is the equilihrium of a gas dissolved in a liquid with the gas above the liquid in a closed system. Carbonated heverages are an excellent example of this type of system. Several articles are available that are written a t the level that the students can understand and enjoy ( I , 2). The students always enjoy discussing their experiences with carbonated heverages and the practical side of chemistry helps them understand why these events occurred. The second way in which I use solution equilihrium a t this unit is in a discussion of a closed system between a saturated solution and an excess of the solute. There are many examples of solution that can he used to show solubility and the various stages of saturation. I often carry out several crystal-growing demonstrations, which are not only very beautiful and fun to do. hut are also illustrations of the concepts heing prwnted. There is aLw a hands-on demonrtration that works auite well to aid student understandine . - .(3). . The effe& of temperature, pressure, and concentration chanees on eauilihrium svstems are also discussed. As this is donerthe importance of ;closed system is again stressed. The
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[HA1 - [H+l [A-I
K.
All too often, however, this equilihrium is presented in the context of the "dissociation of a weak acid," and the pH of a solution of pure HA is calculated as the first example. Historically, that may have been the first application of the concept, hut in contemporary life the master variahle concept is far more useful and important.
' Butler, J. N., "Carbon Dioxide Equilibria and their Applications."
Addison-Wesley Publishing Co.. Reading. MA 1982. 784
Journal of Chemical Education
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a mgl make involvs ms selsnian at msteial to be a, and htims to bedevoted tosach topic. For each column in this re, er, a him school and a college teacher have been invited todiscus why they tee1 a p&imlar topic is impartant and how it conhibutes to hstudents'
understanding 01 chemistry.
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