edited by FRANK SCHIJL; Johnsburg Cenh-al School Nonh Creek. NY 12853
The Titration Project Cary Kilner P.O. Box 595, Exeter, NH 03833 Anyone who is interested in offering a quality laboratory experience realizes that the beauty and elegance of titration is lost on the first-year chemistry student who does only a cursory lab or two. This is what I discovered in my early years of teaching; the students had barely begun to familiarize themselves with the myriad details and suhleties of the titration process, let alone develop any mechanical skill, before we had moved on to a new topic, I noticed, however, that many students who had slogged along up to this point with minimal interest in chemistry suddenly came alive and enjoyed titration immensely. Furthermore, students with a low conce~tualabilitv often had a dormant natural enaineering sense which t6en came to the fore during the proje;t. Bv expandinz titration from a one-shot lab to a more indepth activity %a a sequence of related titrations with a central purpose would give the students time to gain some manudkxpertise and, thus, provide a major new learning tool. In the course of the project, my students acquire the ability to concentrate and to follow directions, organization skill, self-discipline, attention to detail, patience, and an a~nreciationfor error analvsis. Since the ~ r o i e c tcomes in syllabus in May, I take great pleasure in observing their rapt attention no matter how warm and sunnv it is outside! kt Exeter High School we deliver cours& in chemical fundamentals with emphasis on lab so that students will be able to do something k i t h the subject they are studying. During the second semester, we follow the Modern Chemistry (Holt, Rinehart, and Winston) sequence from the kinetic molecular theory, through solutions and electrolytes, into acids and bases, using whole-number pH. The titration project, therefore, tends to he the culminating lesson and experience for the vear and utilizes manv, ~reviouslv learned . skills, especially those involving the mole concept. You will need a collection of volumetric flasks and lots of burets and clamps. These must be set up and left in one place for a t least two weeks. There are no lab partners: each student works independently except a t thevery beginning of the proiect when thev help each other to familiarize themselves with the equipment and the process of titrating, and later when thev prepare solutions in pairs. For weeks prior to the project, we make frequent but vague references t o "the fun last year's classes had", and how i t "uses everything vou've learned so far". Until the students actuallv beein. however, they have no idea what the project entail~,~andihd mvsterv surroundine i t tends to increase their anticipation o i i t g~&iy. In ~ r e ~ a r a t i othev n have Dace references to read and a vocabula~ylist to peruse, though we do not expect them to derive much from this material until thev actuallv start titrating. The project has built into its iniroduction some mechanical exercises and a "quick and sloppy" titration. The goal is to have the students learn what skillful titration is by doing preliminary titrations outside the main sequence. In this way, you can realistically demand and ultimately
my
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80
Journal of Chemical Education
expect precision from your students of +0.003 M in their later runs. The titration proiect consists of 12 Dazes and contains all the informationnec~isary(any lcctureoftext work isstrictly supplemental). The serup checklist has them prepare their workspace and hecome familiar with the buretsby adjusting, filling, and reading levels. In the introductory set, the students are given approximately 0.1 M HC1 and 0.1 M NaOH, then are told that the NaOH is exactly 0.102 M and that they are to find the molarity of the HC1 to three significant figures. Before they start the intro set, but after they have completed the setup check list. I demonstrate one comdete run s o they can see how all the steps on the intro set fit together. The check list for the intro set takes them in detail th;ough one complete run with calculations, which they repeat as manv times as necessary to obtain agreement of 0 . ~ 3 M~. b s studentsget t the hang of it afterihree to five runs; those few who hilve"decided" that thes-can't possihlv do it"soon change their minds as their class&ates (and often their best friends) move quickly and effortlessly on to the main body of the project. They come to understand the necessity for all the detail in order to obtain agreement. In Set A. thev orenare a ~otassiumhvdroeen ~ h t h a l a t e standard sdlution'wiih a "pkdrop" of s-olute (rather than trying to obtain exactly 0.100 M) and calculate its molarity, then use it to standardize 0.1 M NaOH to f0.003 M. In Set B, they find the molarity of acetic acid in vinegar, using their standardized NaOH, and then calculate the mass In Set C, they standardize 0.1 M HCI solution with their NaOH to +0.003 M. In Set D, they find the molarity of ammonium hydroxide in nondetergent Windex with their standardized HCI, then calculate the mass percent ammonia. In both these sets, student results are tabulated on the hoard, and class averages are calculated and used for discussion. In optional Sets E and F, they find the molarity of a saturated solution of Ca(OHL . .. with their HC1 and calculate its solubility (and K,, in advanced classes), and standardize some unknown H2SOa with their NaOH, therebv using normality in both sets of calculations. prediction and testing of pH can also he done. Finally, they tabulate their results and package all their data sheets and calculations into a report in a prescribed format. After such disciplined work, it is always great fun to see their imaginations run rampant as they design covers and booklets for their reports. You can see that students ultimately get practice in cboosing appropriate indicators via transition intervals, (we use phenolphthalein and bromthymol blue for pH 7, phenolphthalein for pH 10, and methyl orange and orange IV for -pH 4), using the Mettler balance, preparing solutions in volumetrics and keeping track of dilution ratio, handling normality, converting molarity to mass percent, and a myri-
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ad of subtle skills and processes that would be impossible to teach didactically. The bottom line is that this is real chemistry, which the students know and appreciate.
A copy of the Titration Project can he obtained by sending $1.00 for postage and a promise to let the author know how it works in your class.
Volume 65 Number 1 January 1988
81
