LECTURE DEMONSTRATIONS in GENERAL CHEMISTRY SAUL B. ARENSON Universitv of Cincinnati, Cincinnati. Ohio
Procedure.-(1) Set up the simple distillation apparatus as diagrammed in Figure 1. Prepare a known solution by taking 50 ml. of chloroform in the 100-ml. graduated cylinder, and adding to i t enough toluene so that the total volume is 100 ml. Transfer the solution to the distilling flask and distil the mixture a t a fairly vigorous rate, catching the distillate in the 100-ml. graduate. Record the temperature as the volume of distillate reaches 5 ml., 10 ml., and so on until 95 ml. are received or until the distillation is complete. (2) Set up the packed column distillation apparatus as diagrammed in Figure 2. Place 100 ml. of the known solution made as before in the Erlenmeyer flask and distil slowly, again recording temperature and volume of distillate. The rate of heating must be controlled to some extent; it should be fast enough so that a drop of distillate forms every second or so, hut not so fast that the tower is flooded and condensed material is unable to return to the boiling flask. (3) Plot boiling point vertically versus volume of distillate. (Figures 3 and 4 are sample curves.) Look up the accepted boiling points of chloroform and toluene and include these as the points'corresponding to 0 ml.
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CONDENSER-
FIGURE 1.-SIMPLE DISTILLATION
FIGURE 2.-PACKED
COLUMN DISTILLATION C
P
RACTICALLY every teacher of general chemistry has many "pet" lecture demonstrations which he has developed for use with his freshman classes. The following were selected from the many experiments sent to me as a result of a letter addressed to teachers in 250 universities and colleges. It is hoped that these contributions will be of sufficient interest and value to encourage other teachers to submit their experiments, for a continuation of this series. 1. DISTILLATION VERSUS RECTIFICATIONL This experiment consists in preparing a known solution of two liquids, and then attempting to separate them, first by simple distillation, and second, by distillation through a packed column.
' Contributed by Gilbert Ford Kinney, Instructor of Chemical
Technology, Pratt Institute.
434
and 100 m1.= The horizontal axis now corresponds to per cent distilled and the purity of any portion of the distillate can he estimated (roughly, of course) from its hoiling point on the vertical axis.
Suggested Questions fm Discussion.-(1) Suppose the. distillate from the simple distillation were divided in four equal fractions as it came over. Estimate the purity of each fraction from its boiling-point curve and discuss what would happen if each fraction were re.. distilled. (2) Explain how i t is physically impossible to do as is often oro~osedbv those with no understanding of the problem, "to hold the temperature half way between the two hoiling points, and catch all of the more volatile liquid as it boils off." (3) Repeat question one for the distillation through the packed column. (4) Compare the efficiencyof the two methods of distillation, and compare the heat requirements of the two methods for equivalent separations.
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2.
EQUILIBRIUM
PROCESSES
IN
WATER
SOLUTION^
In the process of developing the principles of dynamic equilibrium and chemical change involving ions in water solution, i t is obviously good practice to demon2 Ordinarily these points fall on the curve very nicely, but i t might be necessary sometimes to include corrections for thermometer stem, barometer, etc. "his and the three followingexperiments were contributed by Walter R. Camady, Assistant Professor of Chemistry, Reed
strate each type of reaction and each principle separately and to use as many different ions and different reagents as possible. However, after the students have been well introduced to the main reactions and to the important principles, the inclusion of a few more complex experiments, in which a solution may be treated successively with several different types of reagents, may not only arouse considerable curiosity but also promote logical thinking on the part of the student. The following experiment seems to create unusual interest and is thought-provoking. Samples of ferric nitrate, ferric sulfate and ferric chloride are shown to the class and the color of each salt noted. About 20 or 25 g. of ferric nitrate are dissolved in 100 cc. of water and the color of the solution noted. A 20-cc. sample of the solution is saved for reference and to the remainder in a large demonstration tube is added concentrated nitric acid, drop by drop, until most of the color is eliminated. A second 20-cc. sample is saved and the main part of the solution treated with concentrated hydrochloric acid until maximum color is obtained. Following this same procedure the solution is successively diluted, treated again with concentrated hydrochloric acid, with concentrated phosphoric acid, with potassium thiocyanate solution, and finally with a solution of potassium fluoride. Erplnnotion
T*r',fmml
Rerulls
Fe(NOA dinmlued
Solution colored Color faden Color returns Color fades Color deepens Color fades
Fe(OH)s by hydrolysis Mass action, neutralization FeClr molecule colored Increase of ionization Mass action, common ion Complex ion, Fe(FO4--; weak
Deep red color Cplor disappears
F~(CNS)I--ion weaker FeFa--- ion still weaker
HNOx added HCI added Solution diluted HCI added HzFO. added KCNS solution added KF ~olutionadded
3.
VAPOR
PRESSURE-TEMPERATURE LIQUID
CURVE
FOR
A
The apparatus illustrated (Figure 5) overcomes the difficulties usually encountered when demonstrating to beginning students accurately the 'change in vapor pressure of water with the temperature. The evacuated vapor-pressure tube shown comprises a dry side separated from a compartment containing about 1 cc. of water by a "U" of mercury. The wet compartment and the entire "U" is immersed in water contained in an ordinary outer jacket of a Victor Meyer apparatus, which can be heated from below. The vapor pressure is determined by measuring the difference in the height of the two mercury columns. A large bulb is provided on the outside of the water bath. Any moisture that may be carried over to the dry side after several demonstrations may he frozen out by immersing this bulb in a suitable freezing mixture; i. e., solid carbon dioxide and acetone. 4.
BAROMETRIC PRESSURE, VAPOR PRESSURE, VAPOR PRESSURE LOWERING
AND
The apparatus (Figure 6) is similar to ones usually described. However, two modifications, both of which have been found helpful, have been made: The apparatus (tube, support, and bottle of mercury) is hinged a t
FIGURE 6
the bottom so that it may be tipped to any desired angle; also a stopcock and funnel are provided a t the top of the tube so that liquids or solutions may be introduced easily. The barometric pressure, the vapor pressure of a liquid, and the lowering of the vapor pressure may be demonstrated with little manipulation of the apparatus. The tube is filled with mercury by tipping it somewhat and applying suction a t the top from an ordinary aspirator pump, the stopcock bejng turned when the mercury has just reached the top. The tube is locked in the upright position and is ready for the demonstration. The fact that there is a vacuum above the top of the mercury can be demonstrated by tilting the tube until the mercury fills it to the top. The tube is locked in the upright position and the height of the mercury column read. A drop or two of a liquid such as ether is placed in the funnel and introduced into the tube by careful manipulation of the stopcock. The height of the mercury column is again read and the vapor pressure of the liquid calculated. About a cubic centimeter of a solution of oxalic acid in ether is introduced into the tube and the new vapor pressure determined. The effect of the original drop of solvent on the larger amount of solution may be neglected. 5.
DEMONSTRATING BOTH THE LAW OF BOYLE AND THE LAW OF CHARLES BY ONE APPARATUS
The novelty of this setup (Figure 7) lies in the fact
FIGURE 7
that one compact apparatus may be used to demonstrate both laws. It comprises a gas measuring buret (with aspirator flask attached), and an ordinary 500-cc. round-bottomed flask, both connected through a threeway stopcock to a manometer. For obvious reasons the manometer and connecting tubes are made of glass tubing of very small bore. Boyle's Law may be demonstrated by manipulating the aspirator flask, which is filled with mercury, and the stopcock a t the top of the buret until there are about 30 cc. of air a t atmospheric pressure in the 100-cc. gas buret. The gas buret is now connected with the manometer and pressure and volume of the enclosed gas are read. The pressure is reduced by lowering the aspirator flask, and pressure and volume readings are made a t appropriately spaced points until the capacity of the buret is reached or until the aspirator flask reaches the table. In the demonstration of Charles' Law the stopcocks are manipulated so that the central air bulb is open to the atmosphere. The beaker is filled with boiling water and then the top three-way stopcock turned so that the bulb is connected with the manometer. The water in the beaker may be stirred with a thermometer, and readings of pressure and temperature taken simnltaneously a t appropriate intervals as the water bath is cooled. With the use of ice, a temperature range of nearly 100 degrees may be obtained with a corresponding pressure range of about 200 mm. of mercury. (To be continued)
