AN EXPERIMENT ON A BIMOLECULAR REACTION FOR THE PHYSICAL CHEMISTRY LABORATORY
A laboratory experiment upon a bimoleculur reaction, the sapun+catia of ethyl acetate, i s described. The progress of the reaction i s follmed by detamining the change i n the electrical conductien'ty of the mixture with time. A suitable rate of reaction i s secured by running the experiment i n an electric refrigerator. Some typical results are given.
. . . . . .
The laboratory work of the ordinary elementary course in physical chemistry will be found, in most cases, to include an experiment upon a bimolecular reaction. Various reactions will be found recommended in the manuals, and doubtless more are being used in the fonn of mimeographed outlines. The writer ventures to call attention to the followingexperiment, not because of its originality, but because we have found it useful and reliable. During the last eight years we have tried out in our laboratory work in physical chemistry a t Union College a number of experiments on bimolecular reactions. They have been selected from laboratory manuals or recommended by friends engaged in teaching. On the basis of results they have proved disappointing. The standard reaction used as a unimolecnlar reaction, the hydrolysis of cane sugar, bas gone on producing results reliably but the bimolecular reactions have been unsatisfactory. Several years ago I decided to return to the saponification of ethyl acetate by sodium hydroxide, but to use the varying electrical conductivity of the mixture as a measure of the extent of the reaction rather than to titrate the unchanged base as is commonly done. There was nothing original in the idea. I t has been mentioned a number of times.* Experiment soon showed the desirability of reducing the rate of reaction. This was easily accomplished by using an electric refrigerator, a General Electric household size. The temperature remains sufficiently constant, although the cell may be immersed in a beaker of water which, in the refrigerator, remains at a vety constant temperature. We ordinarily place our cell at the bottom of the box, where the temperature is 3OC. The method of conducting the experiment is as follows. A 0.04 M solution of ethyl acetate is prepared by weighing the correct amount of C.P. ethyl acetate, either in a weighing bottle or in a weight pipet. The molarity of an approximately 0.04 M solution of sodium hydroxide is determined by titration against a standard acid. The refrigerator should have been in operation for a day or more. The two solutions are placed in the refrigerator and preferably remain there over night. A clean dry
* Walker has used this type of method in his study of the saponification of esters hy alkalies. Proc. Roy. Sor., 78A, 157 (1906). 513
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JOURNAL OF CHEMICAL EDUCATION
MARCH, 1932
beaker and two clean dry pipets are also placed in the box. The conductivity cell is placed in the box some time before use. Fine copper wires are led from the cell through the door, waxed paper being used over and under them for added protection where the door is closed on them. Best results have been obtained with a closed cell, snch as the Washburn cell, Leeds and Norfhrup No. 4903 type B, although open cells snch as the dipping type Leeds and Northrup No. 4917 have given good results. When the solutions have been thoroughly cooled, portions of each solution are measured into the beaker by means of the pipets and the solutions are thoroughly mixed in the beaker. This is best done in an assay beaker or beaker-flask. The time of mixing should be noted. The condnctivity cell is then filled if of the Washbum type, or the dipping electrode is inserted into the beaker. Resistance readings are taken as rapidly as possible. The experiment from this point consists merely of a series of resistance-time readings. Best results are secured when the students prepare everything on one afternoon and mix the solutions the next morning. Readings are made in our laboratory using an ordinary radio telephone headset, a Leeds and Northrup Wheatstone Bridge No. 4760, and a source of approximately 1000-cycle alternating current. The latter is a t some distance. It consists of an audio-frequency oscillating circuit using a UX-201-A vacuum tube, three large honeycomb coils for inductances, and a paper condenser adjustable in steps. To balance cell capacity an air condenser is used. Readings are taken as required until the resistance remains constant. To be certain of this our students make readings a t intervals for from five to eight hours, and then a t twenty-four and thirty-six or forty-eight hours. The latter will be found to agree. The progress of the reaction is measured directly by the change in conductivity. Data for a conductivity-time curve are plotted and a smooth curve is drawn. The points will be found to give an excellent curve. From several values on the curve, the specific reaction rate, k, is calculated. From the fonn of the equation k
1b(a - 2) = In --(a b)t a(h x)
-
-
it is evident that a better solution will be obtained if a and b are not too nearly equal. Actually, of course, while the value of (a - b) affects the absolute value of k, it will not affect the relative values of k for any one run. If it is desired to make the two concentrations equal, i. e., a = b, which can easily be done by adjustment after standardization of the sodium 1 x hydroxide, the simpler formula may be used. Here k = - -. t a(a - x ) The following are the results of two runs carried out by different students on differentdays. As we have just mentioned, when the values of n and h
VOL.9, NO.3 EXPERIMENT ON BIMOLECULAR REACTION
515
are nearly equal, the effect on the absolute value of k is considerable. The results in each run are in good agreement, however. 3.20 3.00
'",'
2.60
X I
aB 2.20 h a .> ."
2 1.80
P
U
1.40
1.00
Curve 111 5 Curve I1 50 Curve 1 500
10 100 1WO
15 1.50 1500
20 200 2000
25 2.50 2500
30 300 3000
35 350 3500
Time in minutes. n oETHYL ~ ACETATE F m m e I.-COND~CTIVITY-TIME DATAaon m e S a ~ o ~ m c a oa
TABLE I Values of k (Calculated)
Run 1 a = 0.0220 (NaOH) b = 0.02M) (CHCO0C.H.) k
Perrenlagc Chanle
2.05 2.05 2.18 2.19
20 40 fill 80
-
average
2.12
R m2 a
= 0.0221 (NaOH)
b = 0.0200 (CHCOOC?Md
k
Fnrrnlngc Chanw
1.95 1.88 1.i.l 1.79
20 40 GO 80
-
avcrage
1.84
If, as has been suggested, the values of n and b are quite different, such as n = 2b, the results for k in two runs will show much better agreement than is shown between the results of Run 1 and Run 2.
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JOURNAL OF CHEMICAL EDUCATION
MARCH. 1932
The curves for Figure 1 were plotted from the conductivity-time data of Run 1. Three different values for abscissas are utilized in order to show the essential shape of the curve. The student should plot at least curve 111and curve I. On m e I11 he can extrapolate to zero time, to obtain the initial conductivity ( x = 0 when t = 0). Curve I shows the shape of the whole curve, but is useless for an accurate extrapolation to zero time. The experiment may be varied by varying the temperature, two or more runs being made at different temperatures. These temperatures may be secured by a regulation of the refrigerator. A run may be carried out in a thermostat at 20 or 25". The slower reaction, as obtained in the refrigerator has given us very much better results, however, and in our laboratory course we do not carry out any determinations at room temperature. I t is also possible, of course, to increase greatly the concentration of one constituent, in order that its concentration shall remain essentially unchanged, giving a psuedo-uuimolecular reaction. This can hardly be recommended for the ordinary laboratory course in physical chemistry. I t would seem the wisest course, if such experiments are contemplated, to increase the concentration of the ester, because of the relatively large conductivity of the OH.ion and the relatively small change in conductivity which would be produced were the concentration of the base greatly increased. The solubility of the ester limits the concentration of ethyl acetate in the mixture of the two solutions to something less than 0.5 gram mol per liter.
