A LABORATORY EXPERIMENT in GENERAL CHEMISTRY G.E. MACWOOD,E. N. LASSETTRE, AND GLENN BREEN Ohio State University, Columbus, Ohio
A
NEW type of experiment in general chemistry is here described which deals qualitatively with certain prominent chemical reactions involving compounds of the halogen, sulfur, and nitrogen families. There is a need for such an experiment because (1) the principles of oxidation and reduction are first systematically applied at.this point, and (2) because of the wide field which is covered in a limited time. Because of the latter point the experiment is made qualitative. The experiment consists of two parts. In the first part qualitative observations are made on color changes, evolution of gases, and the formation of precipitates when compounds of these families react. In the second part each student is given an unknown to he identified without the use of any chemicals other than those present in the unknown. Nine bottles, each containing a colorless water solution of a pure chemical, are issued to each student. The bottles are labeled with numbers from 1to 9, and along with the set is given a list of the nine chemicals contained in the bottles hut the order of arrangement is not given. The student's problem is to mix portions of the contents of the various bottles, observing the results, and to deduce from these the identity of the chemicals. The chemicals were chosen from the list of eighteen given a t the top of Table 1. The points that were kept in mind when selecting them were-to have (a) common anions, (b) salts that hydrolyze, (c) oxidizing and reducing agents, and (d) one or more precipitants. A list of twenty-one tested unknowns is given in Table 1. A cross in the column headed by a chepical means that this particular chemical is included in the set. On the basis of experience in the laboratory the sets are arranged in the approximate order of their difficulty, the first five being easiest and the last five most difficult. Number 22 will be referred to later. A description of the experiment can best be accomplished by working out an example in detail. Data for set number nine of Table 1 are shown in Table 2. The results obtained when the chemicals are mixed in pairs are given in the lower left-hand portion of the table and in the upper right the results when the chemicals are mixed in sets of three, e. g., when a few drops of bottle No. 3 are mixed with a few drops of bottle No. 5 a white precipitate results. From Table 1 the chemicals present are HzSOa, NaBr, NHEI, KClOa, NaoS, KI, NaOH, BaCle,HzO. Since 8 and 6 give NHa and 8 and 5 give HzS either 8 or 6 must he NH4CI and either 8 or 5 must he Na2S. On mixing 7 and 6 an evolution of heat takes place, and hence must be the result of mixing an acid and a
TABLE 1
base; from the list it follows that 7 must he either H&04 or NaOH and 6 must be either NaOH or H2S04. If 6 were H&04 then, from the preceding statement, 8 must be NH4C1; but H&O4 and .NH,CI do not react to give NHs, hence 6 cannot be H2SOn; it follows that 6 must be NaOH. Also 8 must be NHpCl; 5, NazS; and 7, HzS04. These conclusions are partially conh e d since 7 and 5 give a white precipitate (free sulfur) and H 9 , which is to he expected when H2SO4and NaoS are mixed. Since 7 is H2SOa, we expect a white precipitate of BaS04 when 7 is mixed with BaC12. 0n.examining the table we find that 7 and 3 give a white precipitate, hence 3 must be BaC12. NmS solutions invariably contain SO4- (due to oxidation in air) and hence we expect a white precipitate when 5 and 3 are mixed. From the table we see that this is observed. On mixing 6 and 3 we get a cloudy solution due to precipitation of Ba(OH)2; the precipitate appears only when an excess of NaOH is added. We obtain H2S on mixing 8 and 5 due to the hydrolysis of NH,Cl and Naps, one giving an acid and the other a basic solution. If a small quantity of NH4CIis added to a large quantity of Na2S, the odor of ammonia is perceptible. In the example given, a knowledge of the hydrolysis of these salts is not necessary.
520
There remain to be identified Nos. 1, 2, 4, and 9 which may be either KC103, HzO, NaBr, or KI. Of these chemicals, KC108 acts as an oxidizing agent in acid solution; NaBr and K I are oxidized by KC103 in acid solution.' In the upper right portion of Table 2 are shown the results of adding HzSO4 (No. 7) to all possible combinations of these four chemicals taken two a t a time. 1 7 4 gives.12, and 1 7 9 gives Bra, hence 1 must be KClO3, 4 must be K I and 9 must be NaBr. The only remaining possibility for Hz0 is 2. The chemicals are then identified as follows:
+ +
No. of
~OIUE
I
chrmicol K C I ~
2
HsO
3
BaCh KI NarS
4 5
+ +
NO.of b o r ~ c 6 7
Chamid N~OH Has01
8
NHCI NSB~
s
This example is based upon data collected in the laboratory on an actual unknown. The identification presented here proved to be correct. The experiment has been tried several times with groups of students ranging in number from fifty to a hundred and sixty. From this experience i t became evident that not all of the unknowns are of equal difficulty. On the basis of opinions submitted by various members of the staff, the unknowns are grouped into three classes as stated previously. Number 22 is an unknown so devised that the student determines not only what the arrangement of chemicals is but also what nine of the eighteen. are present. The experiment requires three two-hour laboratory periods, the first of which is devoted to a preliminary exercise on some properties of the eighteen chemicals (first part). The list of chemicals which a sample contains is given out to the student the period before the T h e r e is no danger of KCIOs.
with a dilute solution
unknown is worked and each student is required to present a plan for identification. In order that the student may clearly understand what is required, a demonstration is given by the laboratory instructor in which an unknown is worked. The demonstration requires about forty minutes. A written report is turned in by each student, with chemical equations, in which the steps in identification are presented. All eighteen chemicals are placed in the laboratory in stock bottles so that students may rnn comparison experiments a t any time. In general the number inrorrectly identified may be as high as five. Half of an average class will correctly identify all of the chemicals in their unknown. The unknowns are issued in one-ounce square bottles with rubber stoppers. The stoppers must be free from sulfur in order to prevent sulfur from disssolving in Naps and NaOH. Most satisfactory results are obtained by using amber-colored bottles, to prevent reduction of AgN03, although clear glass is satisfactory if AgN03 solution is prepared just before use. It is necessary to prepare Naps and NapS03solutions just prior to use in order to prevent oxidation by air. The students have exhibited unusual interest in the experiment, which, in the opinion of the authors, affords useful training both in making careful qualitative observations and in making logical deductions based on these observations. It is the authors' experience that students who do not accurately record their observations have vety little success in identifying the unknowns. It is immediately evident that the principle involved can be readily applied to other chemicals than those listed. This particular applicatign is given in detail only because theunknowns have been previously tested. The authors are obligated to many members of the staff for helpful suggestions.
TABLE 2
+ +++
+ +++
---+ +
(a) 1 7 2 - N O reaction (b) 1 + 7 + 4 * I n (c) 1 7 9 Brr (d) 2 4 7 No reaction (e) 9 7 2 No reaction (fl In [from (a)] 7 1 (excess)
No reaction
No reaction
White PPt.
No reaction
No reaction
No reaction
Cloudy
No reaction
No reaction
No reaction
White PPt.
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
1
2
3
4
Tkl
-
colorless solution
reaction
odor of HzS of
Ha
reaction 5
evolved
of NH8
No reaction
reaction
No reaction
6
7
reaction 8
9
