J. N. Cooper Bucknell University Lewisburg. PA 17837
The Protolysis of [Coen2(02CO)]+ A realistic undergraduate kinetics experiment
One of the problems I encounter teaching kinetics in undergraduate physical chemistry is finding reliable experiments that accurately reflect the way kineticists perform solution kinetic investigations. For example, teaching experiments in which the kinetic orders differ from the stoichiometric coefficients are uncommon; those that require a pseudo-fmt-order treatment of an analytical variable other than concentration, or that require analysis of pseudo-order rate parameters to extract kinetic orders with respect to buffered or isolated variables, or that involve the possibility of involving more than one term in the rate law, are quite unusual. I have developed an experiment that illustrates these a p e & in as little as one 4-hr lab period. Moreover the experiment requires only equipment and chemicals that are easily obtainable in modestly equipped undergraduate laboratories. The Experiment
Bisethylenediaminecarbonatocobalt(III) chloride, [Coenz(02CO)ICI. is synthesized readily from Na2C03 and trans[CoenzClzlCl ( I ) , or by air oxidation from cobalt(II) chloride, eth~lenediamineand COz (2). The protolysis of this species in aqueous acids,
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[Coenz(02CO)]++ 2H+ + Hz0 [Caenz(OHz)#+ + COz (1) has been well-characterized (3) and proceeds rapidly and smoothly a t 25'C in the range, 3.5 > p H > 1.0,with essentially first-order rate dependences both on [Coen2(0~CO)+] and on [H+]. The reactants and products both absorb in the visible and near-UV (4) and the kinetics are conveniently followed photometrically over a range of wavelengths, preferably recording absorbance versus time continuously, although point-hy-point data are also acceptable. Solutions of the carbonato complex and a chloroacetic acid-chloroacetate buffer of measured p H are separately thermostatted and mixed just prior to introduction into the spectrophotometer cuvette. Absorbance is recorded as a function of time in a thermostatted spectrophotorneter a t fixed wavelength over at least two half-lives. As the products also absorb light, students are advised to save their solutions to record A, for each experiment a t the end of the lab period. Buffers a t successively lower pH's are prepared and a minimum of four runs a t differing pH's are performed. In their reports, students are asked to confirm that the rate law is consistent with a first-order dependence on the carbonato complex by the linearity of ln(A - A,) versus t plots, where due regard is taken of the increased uncertainty in h ( A A,) for large times. The least squares slopes yield the pseudo-first-order rate parameters, kl(s-'1; students are then asked to plot log k l versus pH in order to infer a plausible, two parameter polynominal to which they attempt to fit their kl, [H+] data. A function of the form
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k1=a+b[HC] a,b>O with a insignificantly different from zero is usually found best to represent the data. Several modifications have been tried in longer lab periods or more extensive reports. (1) The fust-order dependence on the earhonatocomplexan be more definitively established by demonstrating that the k l is independent of the initial [Coenz(OzCO)+].
(2) The temperature and ionic strength dependences of k~ can be determined. (3) The kl obtained by the Guggenheim analysis (5) of the A(t) data can he compared with kl obtained above using measured Am's; this demonstrates the sensitivity of the two data analyses to experimental uncertainty in A(t) and Am,respectively. (4) The optimization of the function fitting the pseudo-order rate parameters to the isolated or buffered variable is by no means trivial; students are encouraged to take more kl, [Ht] data over a wider range of pH and to use statistical tests le.g., Variance ratio, F-test) to discriminatebetween the suitability of alternative fitting functions. Procedure 1) The student is given a sample of recrystallized [Coenz(OzCO)ICl: 70 mg are dissolved in 25 ml distilled Hz0 to give a convenient stock solution. 2, A buffer in the pH range :i.2 to 2.2 gives convenient half-lker 120-22 minj at 25'C; 0.70 g CICHzCOOH dissolved in 25 ml0.20UF NnOH- rivcr s convenient atartine " stock buffer. The buffer's DH i~ determined with a standardized pH meter, e.g. pH = 3.57 for saturated potassium hitartrate, 25% before doing a run. Thermostat the solutions for at least 20 min. The buffer's pH can he altered far succeeding runs without changing the ionic strength by adding 90 mg increments of CICHzCOOH. 3) To begin a m , the spectrophotometer is standardized, and 2.M) ml of each stock solution are mixed in a 10-ml flask, swirled and noured into a waiting cuvette, which is ~ u int the cell holder. The recorder is immediately started and A([) 15 recorded at 511 nm for two or more half-lives;A, for each run is determined ~eparatelyafter seven or more half-lives. 4) At least 4 runs are performed at successively lower pH's and the rate-law determined for the pH range investigated. Question: in changing the buffer pH you maintained constant ionic strength, while changing [CICHzCOOH];what is the kinetic significance of this? How could you vary the pH without ehanging either [CICHzCOOH],the ionic strength or the temperature? What would such an experiment test? Results This experiment has been tested over the last three years with generally enthusiastic student response in a junior physical chemistry lab using a Beckman DB spectrophotometer and Sarnent Welch Recorder e a u i ~ ~ with e d a loaarithmic gear train toconvert poT toahro;h&e. A typical;et of student data obtained in a single 4-hr laboratow are wesented in the table. The kl(calc)\ere obtained from an-unweighted, linear regression,
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k~(calc)= (0.15 + 0.70) X 10-Q (0.94 + 0.05) o ~ + (1) are in reasonahleagreewhere all+ = lo+', and the k~(calc) ment with those cited (3). 1 find it useful, after the lahoratory reports are complete, to refer the students to the literature report (3). both to c o n f m the general level of their agreement and to illustrate the more complex rate law, and hence mechanism, that is obtained when the study is conducted over a far wider range of pH. Volume 57, Number 11, November 1980 / 823
Acknowledgment
Literature Cited
I thank Bucknell undergraduates John Madison and Pam
(1) Sehlessinger, G., ''InorganicLaboratory Preparations," Chcmiesl PuhishingCa.. New Vork. 196Z. p. 230. (21 Springborg. J. and SehAffer,C.. in "Inownic Syntheses," (Editore: Wold,A. and Ruff, J.). MeGraw-Hill, New York, 1373. Vol. X N , p. 61. 13) Jordan, R.and r.mcis.D.. Iharg. cham.. 6,1W5(19611.
Wallace for use of their uncommonly detailed reports in the preparation of this manuscript.
824 1 Jwrml of Chemical Education
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