edited by
GEORGE L. GILBERT Denison University Granville, Ohio 43023
Thermodynamic versus Kinetic Control: A Lecture Demonstration Submitted by:
Checked by:
I. J. McNaught University of Rhodesia P.O. Box MP167, Mt. Pleasant, Salisbury Paul T. Ruda Cleveland Hill Schools Cheehtowaga, New York 14225
Several articles have appeared in this Journal giving organic undergraduate experiments illustrating thermodynamic versus kinetic c ~ n t r o l . ' ,This ~ note describes a simple and effective lecture demonstration of the same concepts. Dissolve 3.06 g HgClz in 225 ml of water in a 600 ml beaker. Dissolve 3.74 e KI in 225 ml of water. To demonstrate to the students t h a t y o u are starting off with the same materials, dilute 25 mi of each solution to 200 ml, the Hg2+solution being in a 600 ml heaker. Start stirring hoth H ~ solutions ~ + with magnetic stirrers then simultaneously add the concentrated I- solution to the concentrated Hg2+ solution and add t,he dilute I- solution to the dilute Hg2+ solution. The concentrated solution immediatelv. . ~ r o d u c e sa hrieht orange suspension while the dilute solution quickly produces a lemon vellow susnension. Leave the solutions stirring during the lectire and let'the yellow suspension slowly turnorang; Ahout 5 min before the end of the lecture switch off the magnetic stirrers and let the red crystals in hoth heakers settle out for audience examination a t the end of the lecture. The concentrations given here give a suitahle result a t the end of a 50-min lecture a t ambient temperature (30°C). The concentration of the dilute solution may need to he increased a t lower temperatures in order to obtain the orange modification hefore the end of the lecture. Successful conversion of the yellow form to the orange form in an acceptable time is verv sensitive to the concentration of the dilute solutions, if theconcentration of the Hg2+is helow about 0.0055 mol d r k 3 the transformation takes loneer than the usual lecture. The relevant equations a r e
(yellow, rhombic) kinetxally controlled product
(orange, tetragonal) thermodynamically controlled product
These equations, of course, represent the stoichiometry and not the kinetics. The relationship between the rate coefficients is" k a > k l i.e., the rate coefficient for formation of the yellow form is greater than that for the orange form.
' McCrew L. A,, and Kruger, T. L., J. CHEM. EDIJC., 48, 400
i1971)
Youssef, A. K., and Ogliaruso, M. A,, J. CHEM. EDIJC., 52,473
(1975) Hine, J., "Physical Organic Chemistry", McGraw-Hill Book Co., New York,1962, p. fig. 722 1 Journal of Chemical Education
The course of the reaction can he depicted on a potential energy diagram as shown in the figure.
Acid-Catalyzed Hydrolysis of Starch Submitted by:
David Blackman University of t h e District of Columbia Mount Vernon S a u a r e C a m ~ u s 1321 H. st.,N.W. ~ a s h i ' n g t o nD.C. , 20005 Madeline P. Goodstein Central Connecticut State College New Britain, 06050 ~~
Checked by:
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Hydrolysis of starch can he demonstrated by adding a small amount of iodine to the polysaccharide, then heating to 10O0C in the presence of an acid catalyst. As hydrolysis progresses the characteristic blue-black color of the starch-iodine complex fades, leaving a solution whose color is just that of diluted iodine. Reagents S~drrhlr~ l a r r h(5%):suspend 5 g of soluble starch in a minimal volume of water; add the s111rwto 50 ml of hoilina water;
cool and dilute to 100 ml.1 Iodine reagent: dissolve 2 g of KI in 25 ml of water; add 1 g of solid iodine. When the latter has dissolved, dilute to 100 m1.2 Hydrochloric acid (4 N): dilute 1volume concentrated HCI with 2 volumes water. Procedure
Mix 0.5 ml iodine reagent with 15 ml HCI in a large (18 X 150 mm) test tube; add 3 ml starch. Heat in a boiling water bath until the color fades completely.
Comments The blue-black starch-iodine complex forms immediately upon mixing. The order in which the reagents are mixed is irrelevant. For convenience, the iodine reagent and HC1 may be combined in advance. with starch added at the beginning of the demonstration. The time required for the dissipation of the blue-black color depends on both the iodine and HCl concentrations. For the procedure as described the reaction time is approximately 2 min. Decreasing the amount of HCl to 20% of that recommended slows the reaction, so that about 8 min are required before any change in the mixture becomes apparent. If the amount of icdime reagent is doubled, approximately 9 min will elapse before the color fades. In any case, however, the time required for decolorization may vary significantly, depending on the age and condition of the starch suspension. Thus it would be judicious to establish, prior to presentation, the exact conditions of the demonstration. ~
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Dr. Madeline Goodstein has pointed out that the system under observation is not as simple as it appears to be. Progressive hydrolysis of starch is known to proceed through a series of intermediates which give varying colors with i ~ d i n e . ~ These color changes are not observed in this demonstration, heat destruction of the iodine-iodide possibly due to reagent. This demonstration is reasonahly easy for an audience to see; visibility, especially in a large room, is greatly enhanced by backlighting.
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'Clark. J. M..,Jr..,and Switzer., R. L.. Biochemistrv." - , "Exoerirnental ,, 2nd E~.,'w.H., Freeman and Co., San Francisco, 1917, p. 161. ZHolum,J. R., Teacher's Manual for "Elements of General and Biological Chemistry," 4th Ed. John Wiley and Sons, New York, 1975, p. 71. 3Holum,J. R., "Elements of General and Biological Chemistry," 4th Ed., John Wiley and Sons, New York, 1975, p. 335. ~
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Volume 55, Number 11. November 1978 / 723
