MISCELLANEOUS EXPERIMENTS CHARLOTTE I. DAMEREL Wilson College, Chambersburg, Pennsylvania
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A demonstration which shows that one may expect two kinds of isomers in the laboratory synthesis of a compound containing one asymmetric carbon atom may be performed very simply with molecular models of the Sargent type. At least 15 students are each presented with the same set of models. A set consists of a carbon-atom ball with four sticks inserted a t tetrahedral angles, and of four other balls of different colors. Different ball groups may be substituted for some of the colored halls. When all of the sets have been distributed the students are told to make modela of molecules by placing the four balls or groups a t the ends of the sticks. These models are then passed to the lecture table and it is seen that there are two kinds of molecules which are mirror images of each other. The larger the number of molecules that can be made the greater the possibility of an equal distribution of isomers.
The use of the most common of salts, sodium chloride, often seems to be avoided in demonstrating the conductivity of fused salts, if one may judge from some laboratory texts and from comments of some teachers of chemistry. Sodium chloride, however, may be melted in a porcelain crucible with the flame from a Bunsen burner if the gas pressure is good and if the quantity of sodium chloride is small compared to the size of the crucible. Our practice is to place a layer of sodium chloride a few millimeters thick in a No. 1 porcelain crucible and to heat the covered crucible in the hottest flame of a Bunsen or Tirrell burner. The salt melts in five to ten minutes. By theuseof tongs the uncovered crucible containing the melt may be quickly raised to platinum electrodes (or any suitable electrodes) in an incandescent lamp bulb circuit, and the lamp glows brightly when the melt reaches the electrodes.
JUNE,1952
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Electrochemical galvanic cells similar in principle to the divided projection cell described by Meldrum1 may be made by using a simple glass U-tube with the two arms separated a t the bottom by a salt agar gel bridge. A U-tube of any desired size may be used, and projection is unnecessary unless the class is to observe an increase in concentration of a solution by the descending stream of liquid from an electrode. Throughout our generalchemistry course we show, as Meldrum describes, that many chemical reactions can be made to occur in these simple cells and that for each reaction the direction of electron flow can be determined by the direction of 'MELDRUM, W. B., J. CHEM.EDUC.,25, 490 (1948).
needle deflection of a galvanometer in the circuit. The students first observe and record the direction of needle deflection of the lecture-table galvanometer when it is in a circuit with a dry cell where the polarity of the electrode connected to each terminal of the galvanometer is known. The repeated use of these galvanic cells m a k e the relation of electrons to chemical reactions much less theoretical than is obtained only from atomic structure and the writing of electronic equations. The students observe experiments which indicate not only that electrons are transferred from one substance to another in many chemical reactions, but also that electrons are transferred in the direction that should be expected from a knowledge of the atomic structure of the elements.
