1. K. Peterson F. H. R I J ~ ~ Y
and
Simon Fraser U n ~ v e r s i t y Burnaby 2, British Columbia, C a n a d a
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A Simple Vacuum Apparatus for Lecture Experiments
W e wish to describe a simple vacuum apparatus for lccture experiments. The apparatus (see Fig. 1) consists of two 1-1 bulbs with an interconnecting 4-mm stopcock arrangement providing stopcock controlled inlets to each bulb. Thus connections can he made to manometers, a vacuum pump, or aspirator, to lecture bottles of gases, and to other glassware via T or Y pieces and stout rubber tubing. Pinch clamps may be used in place of stopcocks hut are generally less satisfactory. The whole should he mounted on a stable portable frame built up from 1/2-in. aluminum rods. While 1-1 bulbs are almost foolproof against implosion, it is advisable to place a clear Perspex screen in front of the equipment and to wear safety goggles, both for protection and to emphasize the need for care when dealing with evacuated vessels. The aspirator should be used to remove harmful vapors (e.g., bromine or organic compounds which dissolve in the oil of vacuum pumps), the system being flushed several times with air. While the 2-3 em pressure limit of most water pumps is adequate for many experiments, an oil pump yielding a "low" vacuum (10-3-10-2mm) must be used if samples are to be condensed from the vapor phase. Demonstration Experiments
Vacuum equipment seems to appeal to students, possibly because it is more elaborate than the simple apparatus normally used, and because it permits the less familiar handling of gases. Experiments were particularly successful with small groups (20-25 people) that were eager to cluster round to be able to observe all operations. By means of TV video-taped projection, however, demonstrations may be presented to quite large audiences. Following are some examples of the use of this apparatus in lecture situations:'
ALYEA,H. N., AND DUTTON, F. B., "Tested Demonstrations in Chemistry," T h e Division of Chemical Education of the American Chemical Society, Easton, Pennsylvania, 1965. FLASK A
FLASK B
Expansion of Gases, Pressure and Temperature Effects, Specific Heats of Gases. Starting with both bulbs containing air at atmospheric pressure, invite a volunteer to read the mercury levels in the manometers. Evacuate bulb A alone, noting new readings. Isolate the pump, and permit the gas in B to expand into A . Again read pressures. Flask A may he evacuated once more, and the operation repeated. Deduce a relationship hetween pressure and volume. A description of the principle of operation of rotary oil pumps is relevant here. Fit B with a small heater element (a loose coil of No. 26 s.w.g. Nichrome, 2-3 ohms), as shown in Figure 2, and connect to a Variac. Using low voltages to regulate heating effects, the tendency for the gas in B to expand, and for its pressure to increase, may he demonstrated. Slowly remove the air from B, to show that the heater element will glow more and more brightly as the pressure is decreased. One of our students wanted to experiment with other gases (wc suggested He and COJ in place of air. Pressure; Density of Liquids. Using manometers with quite different cross-sectional areas (e.g., A, 9 mm; B, 12 mm glass tubing), small amounts of air may he withdrawn from both the flasks to show that the pressure readings are independent of cross-sectional area. Substitute a water manometer for B to show the significance of density. Diffusion; Latent Heat. Place 1 ml of hromine (work in a fume hood) in a phial (Fig. 3), attach at PI, evacuate bulb A , isolate the pump and the manometer A, and open S , and S2to the system. Brown vapor of Br2 rapidly fills the flask, 230HM while the phial of liquid Br, COIL OF NO 2 often cools to the extent of causing condensation. To reverse the effect, cool the phial in liquid nitrogen or an isopropyl alcohol-dry ice 4 LEADS TO VARIAC bath.
II
Figure 2. Hsoting element for the vecuum apparatus.
B 14 GROUND GLASS JOINTS
MANOMETER A Figure 1.
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Diggram of the vocuum apporotur.
Journal of Chemical Mucotion
MANOMETER B
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Figure 3. Phial for the demonstration of diffvriom
With flask A filled with Br, vapor, adjust the pressure in B to about 21 cm, the vapor pressure of liquid bromine at room temperature, and open Sa and Ss. The diffusion of Br2 into flask B is very slow indeed, providing a class demonstration of several weeks duration. Do not ruin a manometer by trying to read pressures! Saturated Vapor Pressure; Diffusion. Using suitable phials (see Fig. 3) containing about 1 ml of liquids such as water, ether (caution), acetaldehyde, petroleum ether, etc., the saturated vapor pressures of these substances may be found. With a phial attached a t inlet PI, flask A is evacuated, the phial is opened to the system, and the vapor pressure is read a t intervals, until equilibrium is attained. Initial cooling of the liquid, resulting in condensation of moisture on the tube, is often observed. Data for other liquids may be obtained in a similar way. Saturated vapor pressures at OO" or -16°C may be obtained by placing ice or ice/salt baths around the phial at TI. Errors due to the presence of air in the phial should be noted. This may be eliminated by pumping off the first two or three samples of "vapor." It is instructive to try to recondense the vapor back into the phial, using liquid nitrogen or an isopropyl alcohol-dry ice bath (-78°C). The presence of quite small amounts of non-condensible gases (N2, 0%) dramatically impedes the diffusion of vapor hack into the phial. Molecular Weight. Attach a phial containing ether (or other volatile liquid, 1-2 ml) at PI, cool in a, liquid nitrogen or an isopropyl alcohol-dry ice bath and remove the air from flask A and from the phial. Remove the phial from the system, warm to room temperature, and weigh. Attach at PI, evacuate, isolate flask A and open the phial to permit ether vapor to fill the flask
(10-20 cm pressure). Read the pressure, reweigh thc phial to find the weight of vapor, and calibrate the volume of the flask by filling with water from a measuring cylinder. The molecular weight of the vapor is calculated, being the weight of STP vapor that would occupy 22.4 1. Heat of Reaction; Reaction Stoichiometry. Using lecture bottles, introduce gaseous ammonia into flask A via PI (e.g., to 30 cm pressure), and a somewhat greater amount of hydrogen chloride into B via P2 (e.g., 40 cm pressure). When Ss and S3 are opened, the gases should mix and react in flask A, producing a white smoke of NH,CI. The walls of the flask become quite hot as a result of heat liberated during reaction. The pressure of unused HCl should be read and the stoichiometry of the reaction determined. There are many modifications to the preceding experiments, all of which require interpretations based on the molecular concept of matter. Without too much difficulty, principles or concepts such as the Gas Laws, molecular motions, degrees of freedom, mean free path, gaseous diffusion, molecular collisions, temperature, specific heat, latent heat, thermal and radiant energy, equipartition of energy, heats of chemical reactions, reaction stoichiometry, Avogadro's Law, constancy of composition, and many other related aspects may he illustrated by or used in the interpretation of such experiments. Acknowledgment
The authors gratefully acknowledge many inspirational discussions with Professor B. D. Pate and colleagues, whose classroom adventures kindle curiosity, excitement, and interest in chemistry.
Volume 45, Number II,November 1968
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