LECTURE DEMONSTRATION EXPERIMENTS* R. D. BILLINGER Lehigh University, Bethlehem, Pennqvlvania
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HERE are always several requirements for a successful lecture demonstration. Among these may be listed (1) the demonstration should explain a definite principle or process; (2) it should be brief in duration; too great a time should not be allowed to (3) the set-up should elapse before something happens; - -
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* Presented before the Division of Chemical Education at the ninety-second meeting of the A. C. S., Pittsburgh. Pennsylvania, September 10, 1936.
not be too complicated; (4) apparatus must be large enough to be seen and arranged for maximum visibility; (5) the experiment should be as fool-proof as possible. This requires selection of workable experiments, a good lecture assistant and careful checking. Three experiments which have been found to meet the above requirements are submitted. The Acid'-Figure shows a diagrammatic sketch of the apparatus used t o
illustrate the contact process for the preparation of sulfuric acid. Sulfur dioxide is prepared in flask ( A ) by dropping concentrated sulfuric acid on a concentrated (40 per cent.) solution of sodium bisulfite. A wash
of glass tubing and broken glass. A bit of glass wool a t the bottom will serve to support the glass packing. The fumes may be drawn slowly through the tower by attaching (J) to suction. From the dropping funnel (G)
bottle;(not shown in diagram) containing concentrated sulfuric acid will serve to indicate the rate of evolution of SOz. The sulfur dioxide passes through a drying tower (D) and then is mixed with oxygen and passed over the boat (E) containing platinized asbestos. The oxygen coming from a gas cylinder is likewise bubbled through a wash bottle (B) of sulfnric acid and a drying tower (C) containing calcium chloride. The boat containing the catalyst is heated in the gas furnace (F) to about 400°C. Under the above conditions almost complete transformation of sulfur dioxide to trioxide occurs. We usually show the class the heavy white fumes which emerge from the silica tube of the furnace. The gases are allowed to collect in a liter flask (not shown in diagram) connected in a parallel line by means of a T-tube a t point T. By adjusting pinch clamps the direction of flow of the sulfnr trioxide can be regulated. Next the white fumes are allowed to pass through the absorbing tower (H). This can be constructed easily by using the outer jacket of a condenser tube filled with short pieces
concentrated sulfuric acid is dropped slowly to absorb the fumes of sulfur trioxide. The fuming sulfuric acid which is formed is collected in the Erlenmeyer flask (I). The experiment serves to illustrate the important steps in the contact process(1) combination of sulfur dioxide and oxygen a t suitable temperatures, (2) absorption of sulfur trioxide in 98 per cent. sulfuric acid. Since so many important principles and applications are described in the lectures on this subject, it seems equally essential to accompany i t with a good demonstration. While the above set-up is somewhat elaborate, i t readily demonstrates the important steps in the process. 2. Preparation of Oxygenfrom Potassium Chlorate.Caution. (The beating of potassium chlorate to prepare oxygen is a lecture demonstration which requires considerable care. In the first place the chlorate must be absolutely free from carbon or other impurities, and the manganese dioxide which is added as a catalyst also must be of high grade. If by any chance a bottle containing carbon instead of MnOn were picked from the
shelf by mistake, a disastrous explosion would result when the mixture was heated. It is well to test the reagents by heating a small quantity of chlorate and manganese dioxide together in a small test-tube prior to any large scale experiment.) The following procedure devised by Dr. H. M. U11mann of Lehigh University has proved successful as a lecture experiment. The apparatus shown in the diagram (Figure 2) is conveniently arranged for the addition of the catalyst to the fused chlorate. A two-liter flask (three-necked)
is used as the generating flask. About six ounces of dry potassium chlorate is carefully heated by means of a Bunsen flame. A wire gauze is used to assure more uniform heating. The bulb of a 500° Centigrade thermometer (Th) is immersed in the chlorate, but kept from touching the hot flask-bottom. B is a glass bucket 3 cm. long prepared from 14-mm. glass tubing. This bucket contains five grams of manganese dioxide. It is held in position by two copper wires as shown. Wire (W) is movable and serves as a release by means of which the bucket can be tilted. When the ring a t the top of (W) is pulled upward the bucket swings mouth downward and spills its content of manganese dioxide. The delivery tube (D) should be made sufficiently wide (12 mm. in diameter) to allow for rapid evolution of oxygen. The flask is slowly heated to about 340°C. before the potassium chlorate appears to melt. At this stage i t can be shown that very little evolution of oxygen is occurring. Now lower the flame slightly and add the catalyst in the method described. The immediate and rapid evolution of oxygen will be apparent and several jars of the gas can be collected in short time. The design of the apparatus obviates the necessity of removing the central stopper during the heating. This prevents any back rush of water from the pneumatic trough to the hot flask. When the liberation of oxygen
has ceased the third rubber stopper is quickly removed in order to establish atmospheric pressure in the flask. The class should be told that complete decomposition of potassium chlorate requires temperatures above ROOo; also that manganese dioxide itself will liberate
oxygen about 400'. By adding the catalyst, however, to potassium chlorate, oxygen can be evolved at temperatures as low as 200'. 3. Demonstration of Vapor Pressure Contrast.-The following experiment is designed to show the lower vapor pressure of a solution of a salt in water compared to the vapor pressure of the original solvent. Figure 3 shows a simple lecture demonstration for this purpose. A small scale (5') is set up under a large bell jar (J). On one pan is placed a beaker (A) of distilled water, while on the other is placed a beaker (B) containing a sufficient quantity of a concentrated salt solution to exactly balance the scale. Sodium thiosulfate is a suitable solute for (B). The edges of the bell jar should be greased and set upon a glass plate (G). In the course of the lecture hour there will be an appreciable change in the position of the scale arms. The side supporting the solution will be lowered, due to an increase in weight. The explanation should show the difference in vapor pressures of solvent and solution. After a time the air in the jar becomes saturated with water vapor and then water distils over from A to B. To obtain as marked a difference as is sbown in the diagram requires a period of several hours, and this can be sbown again a t the next lecture period. Dr. T. H. Hazlehurst of Lehigh University suggested this variation of the experiment, which is described in several texts.
