Richard W. Zuehlke Lawrence University
Appleton. Wisconsin
I
I
The Case of the Unlabeled Bottler Descriptive chemistry in the introductory laboratory course
Several years ago Lawrence University, in common with several institutions, replaced the traditional qualitative analysis course with one dealing with analytical separations. This was offered after a course in ionic equilibria in which the relatively simple equilibria involved in a number of classical quantitative analysis schemes were investigated in the laboratory. It was realized, however, that abandoning the traditional quaLitative analysis course would do away with an important source of knowledge of descriptive chemistry for undergraduates. I n order to compensate for this undesirable effect, the following set of exercises was devised to emphasize the descriptive material without getting a student bogged down in the details of the equilibria involved. The exercise has been used at a number of different points in the introductory chemistry courses, and seems to he equally successful regardless of a student's high school chemistry background. I n the exercise, a student receives seven (or eight) solutions in numbered dropper bottles. He is given a list of important species in the bottles (along with their concentrations), but is not told which species are in which bottles; a given set of solutions is usuauy arranged to contain ions from only one or two qualitative analysis groups. The purpose of the experiment is to identify the contents of each bottle, using the contents of the other bottles as the only reagents. This is accomplished by adding five drops of one solution to a test tube, and observing what happens during the dropwise addition of ten drops of a second solution. Because of the possible existence of slow reactions, the student is also advised to reverse the above procedure, and make ohservations during the dropwise addition of the first solution to the second. He may also mix more than two solutions at a time in order to obtain confirmatory evidence. The student report is expected to contain a tabulation of the observations, a list of the important reactions observed, and a list of each bottle with its contents. A typical example follows. Example
Aside from the general directions, the student is furnished with the following list of the chemical species present: HzO (this could be an only constituent), 3M NH3, 3M H30+, Bi+++, Ag+, Cd++, Hg++, I-, NOa-, SO4-, Cl- (the concentrations in the last group'range from 0.05M to about 0.2M). 1
MACWOOD, G. E., LASSETPRE, E. N., and BREEN,G., J. CEEM.EDUC., 17,520 (1940).
Responses from a typical report are given in Tables 1 and 2. Table 1 shows this student's observations tabulated in the suggested manner; Table 2 gives his list of reactions (mistakes marked with X);Table 3 contains alist of the bottles' contents. Table 2.
-
Equations for the Reactions.
++ +++ + + + + +
-- --
21Hg++ HgL I (red) HgI. 21- + HgL- (colorless) HsOt OH2H20 IAg+ AgI (yellow) 31Bi+++ B& Bi4 I BiL- (brownish orange solution) 2Hgt+ 4 N O s NH4+ OH- -r H1O 3Hf 3 N 0 3 HgzX(NO8) (white) Kg++ SO,- + HgSO, I (yellow) XL Hg++ 2CI- + HgClz I (white) X 20HCdt+ + Cd(OH)?4 (white) X 30H- + Bit++ + 1-I.O BiOOH ( (white) Ag+ C1AgCl1 (white) X 350,2Bi+++ Bil(SO4)s (white) X Cd++ + so.- + Cd++ so4B i t + + + C1- + HnO -t 2Ht BiOCl4 (white)
+ + +
++
--
++
+
+ +
Student Report. Mistakes by the student have been marked X by the instructor.
Discussion
The student is asked to prepare for this exercise by reading about possible reactions between the listed chemical species (several qualitative analysis and inorganic chemistry texts are recommended as references). He is asked to demonstrate a minimum mastery of this material by passing a brief oral examination prior to beginning the laboratory work. Such questions as: "What might he the expected ohservations when mercuric and iodide ions are mixed?" are asked in this examination. Depending upon the general level of class preparation, it may be necessary to provide some additional information in order to allow unambiguous identification of the contents. For instance, the solubility of mercuric sulfate is such that a saturated solution of the salt would not provide concentrations in the ranges listed; also, the solubility would not allow precipitation of the salt when the anion and cation (in separate bottles) are mixed. Therefore, the class would not expect to find mercuric and sulfate ions in the same bottle, neither would they find a reaction occurring when these two ions are mixed. I n addition, this instructor has found that fever enemies are made if students are told that Bi+++is st,abilized in that form by the addition of a small amount of acid to the solution containing the bismuthsalt.. Student response to these exercises has been very gratifying. The exercises are frequently given partly as tension relievers in a laboratory course which emVolume 43, Number 7 7 , November 7 966
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To/Add
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Table 1. 1
The Case of the Unlabeled Bottles.
2 3 Red precipitate No visible would appear effects and then dissolve. Finally
5
4
6
7 No visible effects
Yellow precipitate
No visible effects
Brownish orange solution
No visible effect.?
Yellow precipitate
Light white precipitate
No visible effects
No visible effects
After s. few White precipitate drops s. white precipitate formed White DreciDiCurdy white tate koul;l precipitate form sround the drop and immediately dissolve (moderately sohble salt) White ... precipitate
No visible effects
t~ h e red -~~~ ~ - .
precipitate remained
... Red precipitate immediately formed and after an excess of No. 1the solution cleared White No visible effects precipitate Yellow precipitate
No visible effects
White precipitate
.. .
No visible effects
No visible effects
Yellow precipitate
White precipitate immedi. atdy formed and-then dissolved after 5 or 6 drops of No.
Brownish orange solution
Light white precipitate
~Ihite precipitate
White precipitate would form around the drop and immediately dissolve (moderatelv soluble salt) Milky white DreciDitate
No visible effects
No visible effects
No visible effects
No visible effects
2
White precipitate cleared up with excess No. 5 No visible effects
White precipitate formed immediately and then dissolved after an excess of No. 6 was added
No visible effects
No visible effects
A white precip itate finally formed after 5 or 6 drops of No. 7 were added
* Student's report, John Sawyers.
phasizes the quantitative approach to data taking and interpretation. Their prime virtue, of course, is that the student is forced to employ some knowledge of descriptive chemistry with a definite goal in mind; he does not merely observe, fill in the blanks and call it a day.
Table 3.
Bottle
The Contents Of The Individual Bottles
Contents
Bottle
Contents
Addendum
The similarity between this article and that of MacWood, et al.,' was pointed out to the author by the Editor, but there are some uoteable differences. While the intent of both schemes is essentially the same (to teach descriptive chemistry), the one presented herein is organized around groups of the traditional qua1 scheme so that the differences and similarities in the chemistry of neighboring species in the periodic table are made more obvious. This is accomplished in
602 / Journal o f Chemical Education
large part through the specific practice of not listing anions with cations in the material presented to students. An additional benefit resulting from this practice is that matching of anions and cations can be varied within some limits (unambiguous data must result from the mixing procedure, however); also, a number of entirely different groupings may be made to emphasize any given area of the periodic table.
