E.
Baginski and 0. Zakl Wayne State University Detroit, Michigan
Experiment Demonstrating First Order Kinetics
O n e can easily follow the iodide catalyzed reaction between arsenic I11 (arsenite) and cerium(1V) (cerate) carried out under controlled conditions of time, temperature and acidity ( 1 4 ) . Some work has shown that this catalyzed process follows first order kinetics (4, 5). Other work has endeavored to explain the mechanism leading to these conclusions (6). To go along with these previous findings, details of two experiments are presented. One qualitatively describes the chemistry of the reaction, while the other yields data leading t o a simple resolution of the order of the reaction. This enables the student in the laboratory to conduct a simple kinetic study involving these two essential phases (7). The Experiments Cerium(IV) ammonium nitrate. A 0.02 N solution of (NH& Ce(NOa),was prepamd in a solution 1.44 N in H2S04. Arsenic III acid. A solution was prepared which contained 14.72 g of Na.4802 and 70 ml of concentrated Hi301 per liter. Iodate standard solution. Solution containing 0.0, 1.0, 2.0 and 3.0 mcg of iodine (as iodate) per 100 ml of As 111 solution were prepared from a stock solution of 168.5 mg per liter of KIOa. Mereury(I1) nitmte solution. One gram of Hg(NO& was dissolved in water to which enough HNOawas added to just offset hydrolysis.
Two experiments to describe both a qualitative and quantitative phase were designed. I n the first experiment, a very small crystal of potassium iodide was covered with chloroform in a test tube and then a mixture including Ce(1V)-As(II1) and the correct acidity and volume relations (1 volume of Ce(1V) solution5 volumes of As(1II) solution) were carefully poured above the CHC1,. Two distinct layers were observed,
colorless below and yellow above (Figure 1, left). The tube was quickly inverted once for several seconds and the layers were allowed t o separate. The bottom layer became purple due to oxidation of iodide by Ce(1V) in the aqueous p h h e followed by extraction of the iodine formed into the chloroform phase (Figure 1, center). The upper layer was a fadimg yellow due to reduction of much of the Ce(1V) to colorless Ce(II1). A second vigorous shaking then caused the complete disappearance of the color in both layers as the I 2 oxidized the As(II1) to A@) and the cycle went t o completion (Figure 1, right). From this experiment, one might infer that the IrI- couple, a half cell with an emf between those of the Ce(1V)-Ce(II1) and the As(II1)-As(V) half cells at the acidity employed (S), is capable of catalyzing the Ce(1V-As(II1) reaction in consecutive steps (Figure 1, lower). I n a second experiment, varying micro concentrations of iodine in 5 ml of As(II1) solution were allowed to incubate in a water bath after mixing each with one ml of Ce(1V) solution. At the end of carefully fixed time intervals ranging from 0-60 minutes, the reaction was stopped by introducing a drop of 1% mercury(I1) nitrate solution into the system to tie up the iodide. The unreacted cerium was then titrated with standard iron(I1) ammonium sulfate using 1-10 phenanthroline iron(I1) sulfate as the indicator (9,10). The results graphed in Figure 2 indicate that the quantitative consideration of the reaction rate processes show the over-all reaction kinetics to he of the first order. Titration was selected over spectrophotometry,
' This work was supported in part hy a Granbin-Aid from the Receiving Hospital Research Corporation.
-1.5
0
I
10
I
20
I
30
I
40
I
50
I
60
TIME IN MINUTES Fig. 1 . Qualitative description of the iodine sotolyred CellVl.Arllill reaction
Figure 2. Kinetic study for (A1 0.0, (81 5.0, (C) 10.0, and (Dl 15.0 g/lOO ml ~oncentrationrof iodine. There solutions wore diluted 1:s in anenite inthe final anolydr
Volume 39, Number 12, December 1962
/
635
because a spectrophotometric calibration curve for C e ( W was not linear using the described conditions, whereas titration yielded an accurate description of the amount of Ce(1V) present. Titration was also a much simpler approach than spectrophotometry would have been for the student. Literature Cited (1) ZAK,B., WILLARD,H. H., MYERS,G. B., AND BOYLE,A. J., Anal. Chem., 24,1345 (1952). N.,Anal. Chem., 23,1507 (1951). (2) LEIN,A. AND SCRWARTZ, (3) Foss. 0. P.. HANKES.L. V.. AND VANSLYKE.D. 0.. Clin.
(4) THOMPSON, H.L., KLUGERMAN, M. R., AND TRUEMPER, J.,J . Lab. & Clin. Med., 47 149 (1956). (5) XoNTnxIs, N, E,, AND PICKERING, D, E,, J.Clin,Endorrin, & Metab., 18,774 (1958). (6) YAWS, J. S., AND Taoxas, H. C., J. Am. Chem. Soe., 78, 3950 (1956). (7) GLASSTONE, S., "Textbook of Physical Chemistry," 4th ed.. D. Van Nostrand Camnanv. " , Inc.., New York. 1947. (8) WILLARD, H. H., AND FURMAN, N. H., "Elementary Quantitative Analysis," 3rd ed., D. Van Nostrand Company, Inc., New York, 1949, p. 212. C. M., Anal. Chem., (9) S T R I C K L ~R. N ~D., , AND MALONEY, 29,1870 (1957). R. C., WHITE,E. G., AND ZAK, (10) MEYER,K. R., DICKENMAN, B., Amer. J . Clin. Path.,25, 1160 (1955).
.
-
Creative Writing in Chemistry or "Anything Can Happen" The following is an example of creative writing that recently occurred during a phyaical chemistry examination. While it is not the answer to the question, the essay is a delightful bit of correlation of fundamental principles. The best explanation of this masterpiece is that it was written near the end of e x m i n b tion week when anything might occur. This is the question and the answer as given by Miss Flora. Quesfion
You wish to know the activity of the rhenium ion in an unknown solution. One accurate method of determining this ia by electrochemical means. Design a set of experiments to determine the activity of the rhenium ion electrochemically. Include the experiments1 data you would obtain and how you would use them in your calculations. Assume you are living in the year 1 and n l y the following are known: the charge on the electron, the absolute temperature scale, Avogadro'n number, the gas constant and velocity of light. Clearly indicate any aesumptions you may make. (Note: You do not know the atomic weight of rhenium or the charge of the rhenium ion.) Answer
Known: charge of an electron electron) Avagtgadro's number
=
=
e (therefore, the concept of an
-
N
from this you would have concept of "ground hog," that is the quantity which contains A moleeular, also relative, weight of a "ground hog" of each of the four known elements. some elements and their atomic weights-relative element A element A atomic weight 1 charge element C atomic weight 6A charge element 0 atomic weight 20A charge element X atomic weight 40A charge
to
+l
-2 +l f3
The charge corresponds to the "normal combining power" of them. concept of activity, thus the concept of ions. possible relations concerning activity which might be determined-as suggested by a wise high priest:
En.; Retr
=
E'R~
~ R C * = my+
1. Send slave A (also known as graduate student A ) to make R device to measure the number of "electrons" which pass point B in the "time" it takes 92 grains of fine white sand to fall Call this 1 "finger length" through a hole "2 hairs wide!' "time" unit, 1 gsd. Call the device "electron-meter."
636 / Journol of Chemicol Education
2. Send slaves B, C , D, E , F, G, H, and I with this "electron-meter," which A has made, to pa58 electrons through solutions of A, C, D, and X, and see what happens (do remember to note number of electrons and amount of "what happens.") Results:
Q electrons/l gsd cause s. "ground-hog" of element X to he deposited in a silver dish. '/$ Q electrons/l gad cause R "ground-hog" of element 0 to be deposited. 2 Q electrans/l gsd result in a "graund-hog" of gas C to be evolved (the slave wouldn't confess how he determined this). 2 Q electrons/l gsd result in a "ground-hog" of gas A being evolved. The same wise high priest (knowing where this thing was Q electrons divided headed) suggested that this quantity of by the number of molecules in the "ground-hog" it liberated be known as a fairerday (in honor of the sunnier day they were hoping for). A less wise ~ r i e s added t (when he was transcribing the proceedings) the notion that if one divided the amount "liberated" by this "fairerday" by the "charge" of the element in solution-it was constant in 3 out of the four trials (this was good enough for him) so he called this a faraday. 3. Now slave A noted that the flow of electrons he was measuring caused deflection of a needle on a new "mechanical toy" his son had. This pleased him for it made an easy way to tell if there was a current (or if slave B was taking a coffee break). Now his son gat in the act too and somehow got his "current detector" hooked up between a piece of element 0 in solution with its ions and a piece of element X in solution-and lo and behold, it deflected the needle. Well, he told his dad, who told me. I told the high priest who said, "I knew you would find out eventually, for we need the concept of a current flowing between two half cells if we're ever going to determine the activity of rhenium ion." To date, we have a method for determining the flow of current, its measurement, a time unit, a constant amount of current needed to displace a (ground-bog weight/charge of ion) of "any" ion, and the realization that current does flow eponhneously when a "metal and its ions" are connected with another metal and its ions. Now it remains lo: (1) determine a reference potential, (2) make a "unit activity" standard half cell for RetZ and determine its emf with reference to the reference potential, and (3) determine emf for a half cell of Re, Re+Zof an unknown activity -then apply the wise priest's equation. [And this is where the problem begins.] as told by MISS KATHRYN FLORA to PROP.LEOH. SPINAR' Colorado State University Fort Collins, Colorado Present address: Department of Chemistry, University of Missouri. Columbia.
