edited bv GEORGEL. GILBER; Denison University Granville, Ohio 43023
How to Get the Most from the Dichromate Volcano Demonstration: Aluminothermy SUEMI~TED BY
Wllliarn C. Trogler' Northwestern Universily Evanston. 1U 60201
onstration. Use of a safety shield is advised because of the potential hazard (6)accompanying any thermite process. Acknowledgment I thank L. Carroll King for his help and encouragement.
CKCKEDBY
Kenneth W. Watklns Colorado State University Ft. Collins, CO 80523 The oranee flame and ereen ash from the thermallv initiated decomposition of ammonium dichromate (the dfchromate volcano, eon. (1)) provides a spectacular lecture dem-
..
(5) ..
6th ed., ACS, ~i;.bf hem. & e . , ~ a s t o n , P1965. ~. W... and Barat.. R. 8... J. CHEM. EDUC..66. 675 11979): and referenma
Bazzelli.. J. therein.
(6) Raebuck, P. J.,Edue. in C k m , I78 (November 1979).
This exocharmic (2) reaction illustrates the oxidizing power of Cr(V1) as well as the unusual reducing properties of ammonium ion. I t is not necessary to termin& the demonstration a t this point. The green ash (Crz03) has been used as a catalyst (3) and can be conveniently reduced to the metal. Aluminothermy or the Goldschmidt process (4) usually has been illustrated with the iron-thermite (5)reaction.
Suitable Light Sources and Spectroscopes for Student Observation of Emission Spectra in Lecture Halls S U B M ~ BY D
Elvln Hughes, Jr. Southeastern Louisiana Univwsity
Hammond, LA 70402 CWCKED EV
T h e disappearance of trolley tracks has rendered this particular reaction obsolete; however, it remains one of the best pyrotechnic displays. The Goldschmidt process is used to prepare elemental chromium (usually from FeOCrzOs). As described below, the green ash from the dichromate volcano may also be readily reduced. Crz03+ 2Al- Alz03 + 2Cr
kJfmole(6) (3) Unlike the iron analog, this reaction proceeds smoothly and permils use of a shallow porcelain dish-to contain the rriction. The audience may observe the blinding white heat produced as the reaction spreads across the mixture. At this point emphasize the great affinity of aluminum metal for oxygen, in addition to completing the reduction from Cr(V1) to Cr(0). AH'
= -547.3
Experlrnental Place 20 g of ammonium dichromate (Caution: an eye and lung irritant) in a porcelain evaporating dish and initiate the standard "volcano reaction" with a Bunsen bumer,or with a match. After the dish of Crz03 ash cools, place it on a bed of sand in another porcelain or metal dish. Add an excess (6 g) of Alean aluminum powder (Alcan Metal Powders, Box 290, Elizabeth, NJ 07207) MD 7300 and stir with a spoon to achieve an even color. Simply place a lighted match on the surface of this mixture to ignite. The checker could not s t a t his mixture with a match, but used 2-3 Mg strips to initiate the reaction. Because one initially ignites the excess aluminum powder, the ease of ignition will depend on the activity of the aluminum powder. Alan (MD 7300) aluminum readilv. liehts .. with a match: however. more oarr re mesh sizes may require several .Mg ribhons for ignition. See rrfrrence 16) for a discusion of orher rhermite fusrs.The reonlon will stan slowly (10s)and then rapidly r-5 51 spread. In a darkrnrd room. the emitted white light is rather impressive. At this point the porcelain dish will often shatter and this adds to the effectivenessof the dem-
' Present address: Department of Chemlstly D-006. University of California, San Diego. La Jolla. CA 92093
908
Journal of Chemical Education
Arnold George MansfieldUniversity Mansfield, PA 16933 The b e ~ n n i n echemistrv student often has difficultv in visualizing the ldiscontinukw emission spectra of atdms. Textbooks normally include photomaphs showing atomic line spectra, but these Eannot take theplace of firstrhand ohservations. This article iierks to answer the question "How can students in a lecture hall view spectra?" One can construct a very simple, rugged spectroscope by mounting a transmission grating (600 lineslmm, available from scientific supply houses) to one end of a 24-in.long by 1.5-in. diameter PVC pipe. A 0.001-in. slit is attached to the other end of the pipe. The slit can be constructed by gluing razor blades to a washer with a diameter the same as the outside diameter of the PVC pipe. A piece of paper may be used to aid in spacing the razor blades. The slit assembly is held in place with a pipe coupling as illustrated below. The slit width is not critical: however. a narrow slit width does nrovide ~.~ better resolution of the spectra. The slit muqt be orientxd such that it is oarallel to the mooves on the diffraction eratine. T o view the spectra, aim the slit end toward the emitting source and observe the spectra a t the opposite end. The viewer's eye should be approximately 1in. from the diffraction grating. The spectrum will aonear on either side of the imaee of the illuminated slit. ~ i r s i :and second-order diffraction patterns can clearly be observed. ~~
~
~
~
~
~~~~~
.~
~~~~
- -
CDnshuction of and G. grating.
specbascapes. A. PVC plpe: B, dn assembly; C, coupling;
Six to eight of these spectroscopes can be constructed in approximately three hours and this number should be adequate for a clam to observe spectra during the lecture. Before the lecture on atomic spectra begins, a 175-W mercury-vapor area lamp (outdoor security lamp), a 150-W sodium-vapor area lamp (outdoor security lamp), a 100-W tungsten lamp, and a fluorescent lamp should he placed in the front of the lecture hall. The security lamps are readily available from hardware stores or electricalwholesalers. These sources provide enough intensity for viewing spectra in a lighted room and are housed in a safe enclosure. The students should be instructed to view the spectra from these sources
and to note the differences between discontinuous and continuous spectra. If possibile, also have students observe the spectru&from the sun. If the spectroscopes are constructed properly, one can observe the Fraunhofer lines in the solar spectrum. The Fraunhofer lines are more apparent when viewed in the second order. The light sources and spectroscopes required for this demonstration are rugged and relatively inexpensive. The lieht sources ~rovideenoueh intensitv for stud& to readilv oEserve spectra and to di&nguish detween continuous an2 discontinuous sDectra. thus euhancine their understandine of this phenom&on. ' ~
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Volume 61
Number 10 October 1984
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909
