LECTURE DEMONSTRATIONS in GENERAL CHEMISTRY1 SAUL B. ARENSON University of Cincinnati, Cincinnati, Ohio
6.
DEMONSTRATING THE REALITY OF MOTION AND OF COLLISION OF GAS MOLECULES*
P
REPARE two small (one-cc.) thin-walled glass bulbs ending in a drawn-out tip (similar to a Dumas bulb except for size). Fill these bulbs partially with liquid bromine by the usual procedure of alternately warming and cooling the bulb while the tip is held underneath the surface of liquid bromine. (Caution: Work in a well-drawing hood; do not get bromine on the hands!) Seal off the tip with a flame. Remove the glass stopper from one of two tall (4000cc.) glass-stoppered measuring cylinders, and substitute a one-hole rubber stopper equipped with a glass stopcock. Carefully slide a bromine bulb down to the bottom of each cylinder. Close one cylinder with its glass stopper and the other with the prepared rubber stopper. By means of a mechanical vacuum pump evacuate to about 15 mm. the cylinder equipped with the stopcock. By a quick double inversion of the cylinders, the bromine bulbs are broken as they strike the bottoms of the cylinders. In the cylinder filled with air, the bromine vapor rises only to about one-eighth of the height of the cylinder; in the evacuated cylinder, the bromine vapor almost instantaneously fills (or nearly fills) the cylinder. The deep color of the bromine vapors can be seen by students'in the rear of the lecture room if the cylinders are placed ie front of a white background. Thus the student can observe the upward motion of the colored bromine vapor (i. e., the mass movement of bromine molecules) in both cylinders. In the airfilledcylinder, the mass movement is slow because of the large number of collisions with air molecules. In the evacuated cylinder, where the concentration of air molecules is very much smaller, the mass movement of bromine molecules is very rapid because of this lack of collision. Thus, the demonstration shows (a) that molecules of a gas actually move and (b) that molecules actually collide.
7.
TO SHOW HOW THE DEGREE OF ION FORMATION W A N ELECTROLYTE MAY BE MEASURED^
A tall glass cvliuder of -
small diameter is fitted with
Continued from the September, 1940, *sue. Contributed by W. A. Felsing, Professor of Chemistry, University of Texas. a Contributed by E. L. Quinn, Professor of Chemistry, Universitv of Utah.
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two long copper electrodes extending the whole length of the cylinder. These electrodes are connected to the lighting circuit in series with a large ammeter and a lamp. A little glacial acetic acid is placed in the bottom of the cylinder and a stirring device is adjusted to keep it agitated. When the circuit is closed, no current flows, but as distilled water is added drop by drop, the conductivity gradually increases until the maximum flow of current is obtained. The student can follow the ion formation by watching the ammeter and the lamp.
8. IONIC MIGRATION^ The following demonstration is based on the experiment described in Stieglitz's "The Elements of Qualitative Chemical Analysis." The apparatns, Figure 8, consists of a U-tube of about 16-mm. bore. The total length of the tube from one open end to the other is about 51 inches. It is supported in a suitablemanner and an electrode provided for each end of the tube. The electrodes are made by fusing a platinum wire into &e end of a glass tube of about 5-mm. bore and making contact with a copper wire by means of a few drops of mercury. The tube is prepared as follows: To 250 ml. of cold water, add 12.5 grams of powdered agar agar and 17 grams solid KC1. (The amount of agar agar used may vary from two to five p h cent. The amount given is about right for powdered agar agar, the flake seems to require less.) Stir until well mixed and heat on a water bath for about an hour. Stir occasionally. Clamp the U-tube in a vertical position. Remove the mixture from the water bath and pour enough into the U-tube to fill the bend fn the tube. Place the mixture back on the water bath until that in the tube has firmly set, then remove it from the bath and add four ml. stock phenolphthalein solution (1 g./50 ml. alcohol 50 ml. water). Divide the mixture into two equal parts and to one add five to ten drops of 10 per cent KOH solution and stir until well mixed. This portion should have a nice red color. Holding the beaker containing the red portion in one hand and that containing the colorless portion in the other, pour one into one arm of the tube and a t the same time pour the other into the other arm. The levels should be kept as nearly equal as possible to avoid building up any pres-
+
-
'
This and the following demonstration were contributed by Colonel C. L. Fenton. Professor of Chemistry. U. S. Military Academy.
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sure on the agar agar which has already set. In this way fill each arm to within six inches of the top of the tube. A strip of black paper about 3/16" wide is glued around each arm of the tube a t the top of the agar agar column.
The space above the agar agar is now filled with the following solutions to within an inch of the top. Red arm 1 ml. eonc. HCI 12 ml. CuCh s o h saturated 40 ml. water
Colorlrsr arm 5 ml. 10% KOH 5 ml. water 40 ml. KC1 solo. saturated
The electrodes are now lowered in the solution and an E.M.F. of 110 volts D.C. is applied. *The resistance of the tube as described is about 550 'ohms. At the end of an hour, the red arm below the black strip will have become colorless due to the action of the H i on the phenolphthalein for a distance of about 31/zV. Immediately under the black strip a blue zone about one inch long due to the Cut + will be seen, and in the other arm a red color will have developed due to the action of the OH- for a distance of about two inches. The relative velocities of migration of these ions are thus shown. In an hour's time the tube will become somewhat warm, and if the current is allowed to flow much longer than this the agar agar may soften and rupture. 9. CONDUCTANCE AND IONIZATION The following demonstration has been developed to illustrate ionic equilibrium reactions and the "apparent degree of ionization" and conductance of solutions. The experiment consists of measuring the conductance, as indicated by an ammeter, of a cell (Figure 9)
which contains equivalent amounts of two salt solutions separated by a more or less insulating layer of water. An analysis of the ionic equilibrium diagram for this pair of solutions will indicate the change that will take place in the conductance of the cell when the two solutions are mixed and allowed to come to equilibrium with each other. The solutions are then actually mixed and the change of conductance, if any, noted on the ammeter. Since the values of conductance noted are only relative, the various factors which would affect the conductance of a cell of this kind are limited, for all practical purposes, to the total number of ions between the electrodes a t any given time. The apparatus required consists of a conductivity cell of a type similar to that shown in Figure 10, a D.C. ammeter (0-1 ampere; a lecture table galvanometer with a one-ampere shunt is excellent), a source of D.C. voltage from 8 volts to 120 volts, and a separately funnel of 250 ml. capacity, stirring rod, solutions, etc. A separate cell will be required for each pair of solutions used; usually a number of cells are connected in parallel and the ammeter is placed in the supply line so that any cell may be placed in the circuit a t will. The cell shown in Figure 10 consists of a glass battery jar of rectangular shape about 2" X 5" X 6'/%" deep mounted on a wooden base about ll/lw X 6" X 61/r''. A recess is mortised into the base 5 / s v deep in which the jar fits. Binding posts and a switch are mounted on the base. The permanent wiring of the base terminates a t two wood screws, with washers under the heads, one a t each side of the base block-at the ends of the jar.
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COPPER ELECTRODES
UPPER SOLUTION
LOWER SOLUTION
OCTOBER, 1940
471
Connection is made to the electrodes by a flexible lead which is soldered to the electrode and has a lug which slips under the screw head a t the other end. A twist of a screw-driver and the jar with the electrodes can he removed from the base for cleaning. The electrodes are made from 20-gage sheet copper cut to the width of the glass jar and long enough so that the upper end can be bent over, forming a clip to hold the electrode in place. The electrode thus covers the entire end of the cell. use the seven pairs of solutions shown in the following table. There are undoubtedly others which could be used, but i t is believed that these seven bring out most of the important points.
we
ond pair (NHCl and NaOH) the following diagram is used: 1
%
84%
N K C I q N H ~ + 4- Cl-
+ + N a O E ~ O H - + ??a+ N&OH LH
98.7%
70%
91%
NaCl 111
16%
From this diagram it is concluded that there are fewer ions between the electrodes of the cell after mixing than
Rrrulr of Pab
Umar
No.
mlulsan N/10 KC1 N/10 NHdCI N/10HCI N/10 HA= N/10 NaOH
1 2
3 4 6 8
7
Lower ~~Iulion
N/10 NaNOa N/10 NaOH N/ID NaAc N/10 N&OH N/10 HCL N/40 ~gaor* N/40 BaCIz* Glacial HA=
Water
* upe distilled water and carbon electrodes.
B.M.B.
wad 12 v.
18 v. 8 v. 48 v. 8 v. 72 v.
80 v.
lniring on condvdiaily None Decrease Decrease Increase Decrease Dccreare to zero Increase
A quantity of colored weighted water will be required and is prepared as follows. One gram of red aniline, or other water-soluble dye, is dissolved in about 500 ml. hot water and the resulting solution diluted to two liters. Five hundred grams of ordinary cane sugar are added. Three hundred ml. of each solution (except pair No. 7) are poured into suitable containers and 150 grams of sugar are added to each of the solutions listed as "lower" in the table. In the case of pair No. 7, 100 ml. of glacial acetic acid are placed in one container and 800 ml. of the colored water prepared above are placed in the other. The cells are filled by first pouring the "upper" solution into the jar, inserting the separatory funnel, and, with the stopcock open, applying suction to the mouth of the funnel until the stem is completely filled with solution. If any excess solution is drawn up into the bulb of the funnel it must, of course, be allowed to return to the jar, hut the stopcock must he closed before any air enters the stem. One hundred ml. of the colored water are now poured into the funnel and the stopcock opened a little. The colored water will form a layer under the "upper" solution and raise it as the jar fills. The stopcock must be closed before any air enters the stem. The "lower" solution of the pair is now poured into the funnel and the stopcock opened as before until the bulb is empty. Again, the stopcock must be closed before any air enters the stem. If it is found necessary to add liquid to the funnel bulb the stopcock should be closed while this is done, as otherwise air bubbles will get into the stem of the funnel. The funnel is now carefully removed from the jar and the cell presents the appearance shown in Figure 9. In use, an analysis of the corresponding ionic equilihrium diagram is first made, e. g., in the case of the sec-
FIGURE10
before and therefore the conductance will he less. The appropriate voltage is applied, according to the table, the switch closed and the ammeter reading noted. The two solutions are then thoroughly mixed and any change in the ammeter reading noted. Cell No. 7 consists of a thin layer of glacial acetic acid over 800 ml. of colored water. Upon mixing the conductance of the cell is, of course, increased. Cell No. 6 requires a little special consideration. As can be seen, the end-products are practically insoluble and therefore the conductance drops close to zero. In order for this to he shown successfully (1) carbon electrodes must be used instead of copper; otherwise the silver will deposit and the copper go into solution while the cell is standing; ( 2 ) exactly equivalent amounts of the two solutions must be used; and (3) allowance must be made for the fact that in removing the funnel from the cell a considerable amount of the lower solution is also removed.
In most of the solution pairs it will probably not make much difference which solution is made the lower one, hut i t is quite necessary in the case of pair No. 6 that the BaCL solution be made the lower one, because the sugar will quickly invert and reduce the silver if it is added to the Ag2S04solution. It has been found impractical to illustrate some of the "borderline" cases not only because of the small change in conductance involved but because of the effect of the sugar on the conductance of the solutions. For example, the conductance of a N/10 NaAc solution, measured with a conductance bridge, was found to be more than three times that of a similar solution consisting of 300 ml. of N/10 NaAc to which 150 grams of sugar had been added. Consequently, unless the change in ionic conductance is considerable, it is apt to be masked by the effect of the sugar. 10. IODINE ON TURPENTINE^ Take about 10 grams of solid iodine, crushed just This and the following demonstration were contributed by John D. Clark, Professor of Chemistry, University of New Mexico.
a little in a glass mortar, and with care pour on around five to ten ml. of turpentine. This demonstration came to me as follows: I visited a farm in the Rio Grande valley. An old-time "horse doctor" was there just ahead of me. A horse had a very large abscess. The "doctor" worked crystals of iodine into the sore and poured on turpentine. The farmer was impressed. So was the horse. 11.
DIFFERENT RANGES OF EXPLOSIBILITY OF AIR-GAS MIXTURES
I find a toy calcium carbide cannon useful in showing (very roughly) the different ranges of explosibility of air-gas mixtures. In our large lecture room we have natural gas (methane, mostly). It is difficult to run in gas from a burner (cork in muzzle of cannon) and get an explosive mixture (5 per cent to 13 per cent in air). Many attempts result in failure; indeed, failure is the usual result. Water and carbide result almost always in an explosion the first time the attempt is made with the acetylene-air mixture, since its range is three per cent to 73 per cent. (To be continued)
