The Photochemical Reaction of Hydrogen and Chlorine: A Lecture ~emoAtratYon CARL M. FURGASON1and JOHN W . MOORE1 University of Texas, Austin, Texas
M
OST PHOTOCHEMICAL reactions proposed as lecture demonstrations have the disadvantages (a) that the reaction must be followed by analytical procedures such as titrations, as in the photobromination of cinnamic acid or (b) that a light source of extremely high intensity, such as quartz capillary mercury vapor lamp, may be required, as in the photolysis of uranyl oxalate. The effect of light on the reaction of hydrogen with chlorine has been studied for more than a century, the explosion resulting from the exposure of a mixture of these gases to sunlight having been used a t times as a lecture experiment. The hydrogen-chlorine reaction may also be used as lecture demonstration to illustrate the effect of light of various frequenciesand intensities on chemical changes. The light source may be a large projection lamp, a photoflood lamp, or a high intensity ribbon filament lamp (used in photomicrography). The reaction may be followed by observing the change in the volume or pressure of the gas in the reaction chamber. A mixture of chlorine and hydrogen is placed over water in a test tube (see A in diagram). When the tube is exposed to light, the reaction H&) + Cl&) + 2HCl(g) 2HCl(disrolved) takes place. As the hydrogen chloride dissolves in the water, the volume of the gas space in A is decreased, causing water to enter from tube B and thereby decreasing the pressure in B. A projection manometer2 (J) is connected to tube B. The change in pressure is shown manometrically on a screen and may be observed readily by a large class. Nemst3 has proposed the following mechanism for the reaction: Cl* CI* (chain-startin8step) C1, + hv
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When the cell is first exposed to light, there is an induction period during which certain impurities, such as ammonia, undergo reaction and the newly formed hydrogen chloride molecules condense into a fog of hydrochloric acid which is slowly dissolved in the confining water. When the light source is returned after an interruption, there is an observed increase in volume, due to heatinz of the ws. After a few seconds, the 1 Present address: Tennessee Valley Authority, Wilson Dam.
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volume decreases because of reaction and continues to deaease until the light source is removed or one of the reagents is used up. When the light source is removed, there is a sudden decrease due to cooling of the gas, followed by a very slow deaease in volume due to the completion of solution of hyd+ogen chloride molecules. The more intense the light source, the greater will be the number of photons absorbed per second, and the greater the observed rate of reaction. It will also be noted that the temporary increase in volume a t the beginning of each exposure of the cell to light is greater when using a more intense light source. The energy required for breaking the CI-41 bond corresponding to light having a is 57.8 kio~alories,~ wave length of about 4944A. Accordingly. light of longer wave length should not initiate the reaction chain. This may be demonstrated by placing a piece of red glass or red cellophane over the source of light; the reaction will decrease markedly or cease entirely. depending upon the transmission characteristics of the red filter used. On the other band, a piece of blue glass or blue cellophane may be placed over the light source without appreciably decreasing the rate of reaction. 'MOOREAND FURGASON, "Simple projection manometer for lecture demonstrations." J. CXEM. EDUC.. 19.613 (1942). N.ERNST, "Zur Aukendung des ~insteidschen'photochemischen Aquivalentgesetzes. I," 2. Elektrochcn.. 24, 3354 (1918). "Nature of the chemical bond." Cornell University PAWLING, Press, Ithaca, New York, 1940, p. 49.
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ASSEMBLEDAPPARATUSFOR DEMONS~UTION OP PHOTOSYNTHESIS O P HCL
Because of its high energy, ultra-violet light will accelerate the reaction markedly. (Caution: the ultra-violet light source should not be held too near the tube, nor should the light be of too high an intensity.) PREPARATION
Chlorine is quite soluble in water. To avoid false effects produced by the solution of chlorine during the experiment, the liquid used in tubes A and B should be saturated with chlorine. Oxygen inhibits the reaction. Therefore tube A should be filled with chlorine and hydrogen in such a manner that no air is admitted. This may conveniently be done by filling tube A and the tubing leading to it with saturated chlorine water. Tube C is then connected under water to the delivery tube of a chlorine generator or to a tank of chlorine. Chlorine is allowed to flow into the tube to an extent of about
one-third its volume. The tube C is then connected (again under water) to the delivery tube of a hydrogen generator or to a tank of hydrogen. The tube is now almost filled with gas; a layer of saturated chlorine water about one cm. deep is left in tube A . Mixtures of chlorine and hydrogen explode instantly on being exposed to direct sunlight or to any other extremely intense source of blue or ultra-violet light. While the pyrex glass tube A will filter out all incident light of wave length less than 3200 A,, care should be exercised to avoid exposure to sunlight. '.For this reason tubes A and B are enclosed in a pasteboard or wooden box (F). ACKNOWLEDGMENT
The authors wish to express their appreciation to Dr. W. A. Felsing for his assistance in preparing the manuscript, and to Mr. Arthur Patureau for making the drawing.
