VOL. 5. NO. 10 LECTURE DEMONSTRATIONS WR GENERAL CHEMISTRY
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LECTURE DEMONSTRATIONS FOR GENERAL CHEMISTRY C. HARVEY S O R ~UNIVERSITY , OF WISCONSIN. MADISON, WISCONSIN
EQUILIBRIUM AND MASSACTION Principle Illustrated.--Cobalt chloride probably exists in solution in the hydrated form, the color of the solution varying with the degree of hydration as indicated by the following equation:' CoCI2.xH20(red)
CoCI,.(x-y)H,O
(blue)
+ yHIO
At low temperatures the solution is red, due to the predominance of the highly hydrated red form. As the temperature is increased the equilibrium is shifted to the right and the solution becomes more and more blue in color, due to the predominance of the less hydrated blue form, i. e., due to the partial dehydration of the highly hydrated form. Since the change in color is due to a shift in the point of equilibrium, i. e., to a change from the highly hydrated red to the less hydrated blue, and since this shift is the result of dehydration, it should he possible to bring about a color change not only by increasing the temperature hut also by adding a suitable dehydrating agent. Such a shift of equilibrium, b t , by change of temperature and, second, by the addition of a dehydrating agent, can be demonstrated with the following experiments, both of which lend themselves readily to use in the lecture. Experiments and Observations.-(1) Shift of equilibrium with temperature. Place about 200 cc. of a cold saturated solution of cobalt chloride in a test tube. The color is a cherry red. Pour about 150 cc. of this red solution into a second tube and heat to boiling. The color has changed to a deep blue. Pour the blue solution into a special test tube made by sealing off one end of a two-foot length of large glass tubing. Immerse this tube in an ice or ice-NaC1 bath to a depth of about six inches, keeping the upper part warm by means of a Bunsen burner. The lower end will soon become red while the upper part will remain blue, illustrating, in a manner quite striking and impressive to the observer, the shift of equilibrium with change of temperature. (2) Shift of equilibrium due to removal of water. Place about 200 cc. of a cold red solution of cobalt chloride in a convenient test tube. Add several pieces of anhydrous calcium chloride. The lower portion of the solution, with which the calcium chloride is in contact, soon turns deep blue while the upper part remains red. The color contrast is pronounced; and the result illustrates, quite convincingly, the shift of equilibrium due to a decrease in concentration of one of the products. Remarks.-Since the student will already have been familiarized with ' Howell, J. Chem. Soc., 1927,158.
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the facts relating to hydrated salts and the dehydrating property of anhydrous calcium chloride he should have little difficulty in seeing the application of the principles involved in this set of demonstrations. The fact that the same substance is used in both experiments should help emphasize the point that a given equilibrium may be shifted in more than one way.
Principle Illustrated.-As a general rule the solubility of a solid in a liquid increases with increase in temperature. There are numerous exceptions t o this rule, a notable one being that of cerium sulfate whose solubility in water decreases from 19.09 a t O°C. to 4.78 a t 5G°C. to 0.78 a t 100°C. The decreas? in its solubility with increase in temperature is so great that i t can readily be demonstrated as follows: Experiment and Observations.-Place 100 to 500 cc. of an aqueous solution of cerium sulfate (Ce2(SOa)J,saturated a t zero, in a round bottom flask. Heat to boiling. A mass of crystals will precipitate out. Rmarks.Since cerium sulfate dissolves very slow;y in water a t low temperatures the experiment does not lend itself to the illustration of the increase in solubility with decrease in temperature.
