A TEMPERATURE-SENSITIVE STIRRING ROD Liquefaction of NO, as a Student Experiment ROBERT D. EDDY and SAMUEL R. SCHOLES, Jr.' Tufts University, Medford, Massachusetts
A N Y O N Ewho
has ever laboriously prepared a Liquefied gas-then watched his product evaporate while he frantically tried to seal it in glasswill remember the incident as particularly frustrating. One day, succumbing to a diabolical urge to share this experience with our students, we devised a laboratory exercise which turned out t o have considerable merit. It is a real test of laboratory skill, engaging the attention of the very best st,udents. At the same time it is tantalizing enough to interest even the most indifferent of their classmates. It requires little by why of equipment. I t illustrates visually a number of important points about distillation, condensation, and storage of gases under pressure. It provides laboratory experience with oxidation-reduction reactions of nitric acid and metal nitrates. Finally, because of the well-khown dimerization of NO2,the student provides himself with an interesting little conversation piece, a stirring "rod" which indicates the approximate temperature as he uses it to stir hot water, cool water, or ice water. The expekiient is quite flexible, for the instructor may change its length, not only by changing his standards of what constitutes an acceptable product, but by starting at differentpoints in the preparation. If hoods are available We class may begin nith lead and nitric acid as described here. If hoods are not a t hand, dh. if the period is brief, a small amount of previously dried lead nitrate could be issued. For grading purposeS, the instructor can easily determine the purity of the student's final pro.Uuct (freedom from both solids and water), simply by noting the appearance and color.
DIRECTIONS FOR THE EXPERIMENT Nitrogen(1V) oxide, NO2, is a red-brom corrosive gas with a characteristic odor. Because its boiling point is close to room temperature, the gas is easily liquefied and may he safely stored in sealed glass tubes. At lower temperatures, two molecules of NOz assoriate Present address:
Alfred University, Alfred, New York.
VOLUME 35, NO. 10, OCTOBER, 1958
to form a colorless dimer, dinitrogen tetroxide, N204. Although the gas is easily made by the thermal decomposition of heavy metal nitrates, certain precautions are necessary if a pure product is to be prepared. Kot only must the chemicals and apparatus he thoroughly dry, but because the XO1 readily attacks cork or rubber stoppers and rubber tubing, the apparatus must be made entirely from glass. Preparalion of Lead Nitrate: Place about three grams of lead in an evaporating dish in the hood, and treat with dilute nitric acid. Heat carefully, keep the mixture will stirred, and add frmh acid from time to time as is required to keep the reaction vigorous. When the la?t of the lead has dissolved, remove the excess acid and the water by evaporation. Stir the wet paste constantly to avoid spattering. Do not overheat, for excessive temperathre will decompose the solid. Be certain that the final product is dry, for the purity of the NOz to be made will depend upon the care with which this evaporation is carried out. Preparation of Receiver: Completely seal one end of a 25-cm. length of 8-mm glass tubing and constrict the tube slightly a t a point about 15 cm. from the sealed end. If desired, a small bulb may he blown on the sealed end, but the walls of the bulb must be kept thick. Any moisture condensed in the tube during the blowing operation should be carefully removed. Preparatim of Generator: Place the dried lead nitrate at the bottom of a soft glass test tubeZkeeping the solid from coming in contact with the side walls by means of a rolled paper. Be sure that all paper is removed before heating. Holding the test tube by the ends, rotate the center in the hottest nart of the Bunsen flare. keeping an angle of about' 20" with the horizontal: When the glass is thoroughly softened, pull the ends 60 cm. or more apart with a rapid, steady pull. Let These test tubes must have reasonably thick walls. Expe~ience has shown that the lieht-weieht thin-walled variety cool off too fast t o be satisfactorilv drawn out. The lead salt should be kept off the surfaces to & mftened, because the lead oxide formed on heating dissolves in the glass and weakens it.
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the glass cool, and break off the capillary close to the open end. Now make a bend in the capillary, close to the point where it joins the test tube. Use the smallest possible burner flame. Grasp the test tube firmly with fingers of both hands t o hold it as steady as possible. Point the capillary end upward a t an angle of about 45",and pass the point t o be bent slowly over the flame at a considerable height above it. During the instant that the thin glass is softened, gravity mill provide the needed bending force.3 Preparation of NOa: Insert the capillary tip of the gznerator to the bottom of the constricted receiver tube. Set the receiver upright in a graduate or large beaker and surround it with an ice-salt mixture. Clamp the generator in position; and begin heating it gently to produce NOt. When a sufficientvolume of liquid has collected in the receiver, stop heating the Withdraw the capillary, carefully. Sealino the Receiver: With the lower end of the constricted tube still immersed in the ice-salt mixture, seal the tube off a t the constriction. This is best done by holding the tube steady and moving the burner, applying the flame evenly to all sides of the constriction. Grasp the tube a t the open end with tongs, and heat until the glass collapses. Draw off the upper portioll The instmalor who has learned the knack can make a generator in front nf the class in about a minute and a half. After being shown lulw "easy" it is, his inexperiencej students d l work, seemingly for hours, trying to duplicate the feat.
RECEIVER \
i
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and allow the seal to cool thoroughly before removing the tube from the ice-salt bath. This operation has to be done right the first time, as there is little chance for success o n a second try. When thoroughly cool, use the sealed tube to stir samples of hot water, cool water, and ice water. Note and explain any significant changes.
JOURNAL OF CHEMICAL EDUCATION
