edited bv: RON DELORENZO Middle Georgia College Cachran. Georgia 31014
An Analogy tor the Leveling EItect in Acld-Base Chemistry F. Axtell Kramer St. Louis Communiw College at M e r m 11333 Big Bend Blvd. Kbkwood. MO 63122
I have found that students accept readily enough the idea that different Bronsted-Lowrv acids differ inherently in their abilities to donate protons. But the students havehad difficulty understanding how a given solvent might "disguise" that difference for some acids and not for others. Thus, our general chemistry students have appreciated the following analogy of protons to bricks and seem t o understand the concept of a leveling solvent better because of it. The students are given the following description of three different bricklayers. The first bricklayer is inherently capable of laying 1000 bricks a day on a brick wall. The second bricklayer is capable of laying 800 bricks a day on a brick wall. The third bricklayer is capable of laying only 500 bricks a day on a brick wall. (These figures are intended for ease of illustration and are not meant to be realistic!) Then the students are asked to rank these bricklayers in order of inherent bricklaying ability. They do so; I then tell them about a Union Work Rule that defines a day's job as 700 bricks laid on a brick wall. Next I ask them, "How many bricks a day does the 1000bricks-a-day bricklayer lay?" I repeat the question for the 800-bricks-a-day bricklayer. The students answer, "700 bricks of course; they do not need to do more (and are not allowed to do more) even if they are able to do more." Finally I ask, changing the question from its expected form, "What kind of bricklayer is the 500-brick-a-day bricklayer?" By this time the students have caught on and answer, amid laughter, "A weak acid bricklayer!" T o emphasize the point, I ask the students what would
need to be done to "level" the actual bricklaying "strengths" of these three to the same number of bricks a day laid, and what change in conditions would allow us to differentiate between the first and second bricklayers. Achemical analogy in Bronsted-Lowry terms then follows from this "real-life" situation! The three acids that I use in my presentation are perchloric (inherently the strongest of the common mineral acids), hydrochloric (also a strong acid in water, but an acid that is inherently weaker than perchloric), and acetic (familiar to the students as the acid of vinegar). The solvent water is the work rule that makes the first two acids behave equally strong a t the same nominal concentration. Acetic acid is unable to donate protons (lay bricks) to the capacity that water requires for complete dissociation (a day's work) and so behaves as a weak acid. I then suggest that we revise the work rule downward. We can make it easier to lay sufficient bricks (donate sufficient protons) by choosing a solvent that requires less able bricklaying for complete dissociation (a day's work). The solvent ammonia, as a pure liquid, is not familiar to most of the students, but they readily accept that it is more basic than water and that i t would accept protons more easily. Thus ammonia allows complete proton dissociation (strong acid behavior) by an acid (acetic) that is inherently weaker than the first two examples I used. In terms of the analogy, ammonia has provided a less severe work rule that is, therefore, within the capability of inherently less able bricklayers. For a solvent to be a differentiating solvent, the work rule needs t o be revised upward from its first value. The students soon see that this requires a solvent inherently less basic than water. This solvent would require such capable bricklaying (proton-donating ability) to achieve the requirements of a day's work that only the best proton donors could cause complete dissociation and be, therefore, classified as strong acids.
Volume 63 Number 3 March 1986
275
