edited bv
tested demonstrcrtions Augmenting a Classical Electrochemical Demonstration Submilied by Susan M. Yochum and John R. ~uoma'
Cleveland State University Cleveland, OH 44115 Checked by
Wayne L. Smith
Coiby College Watervilie. ME 04901 Students find electrochemistrv difficult to master because they cannot observe or imagine what happens on the microscopic level i n a n electrochemical reaction. I n a n attempt to address the learning styles ( I ) of our students and to teach electrochemistry in a concrete manner, we have augmented and amplified a classical demonstration (2, 3). Method: Classical
I n the classical demonstration, students are shown three reactions. First, zinc is placed in dilute HCI. Students observe the evolutionofbubbles, hydrogen gas, a t the surface of the zinc. Second, copper is placed in dilute HCI. Students observe that no gas evolves a t the surface of the Cu. Lastly, the Zn and Cu are connected and placed in dilute HCI. Students observe the discrepant event! Bubbles, Hz(g),evolve from the Zn and the Cu. Method: Augmented Part A
The following reactions are presented to the students, or a video2 of these reactions is shown to the students. Reaction 1. A 5-cm strip of metallic Cu is placed in a 100-mL beaker containing 40 mL of 0.1M HCI. (No bubbles appear on the Cu.) Reaction 2. A 10-cm strip of Mg ribbon is doubled and placed in a 100-mLbeaker containing 40 mLof 0.1 M HCL3 (Mg reacts vigorously with the acid and gas bubbles develop quickly on the Mg surface? ) Reaction 3. Cu and Mg strips of the saine size are connected and placed in the 0.1M HC1 a s shown in the figure. (Students observe the first discrepant event! Bubbles evolve a t both the Mg and Cu surfaces.) In a short period of time, all of the immersed Mg reacts. To cause the immersed Mg to react even more quickly, one 'Author to whom correspondence should be addressed. 2A demonstration video avoids the dangers of and reduces many of the time demands of a live demonstration. The authors made the video with a VHS camcorder. 3Hioherconcentrations.such as the commoniv used 3M. cause the reaclnon to proceeo loo raplaly 'Tne amomt of hyorogen gas generated s no1 nazardo~sA so~s of .tons can be d sposed of oown the dra n * th c o p ~ o amodnts water. SCloudinessof the solution occurs immediately. A white precipitate settles to the bottom of the beaker later. 6Asu~ey requiring a written student response was administered at the end of the term. 0
GEORGE L. GILBER~
Denisan University Granville, OH 43023
can add 3M HC1. When all of the immersed Mg has reacted, the students observe t h e second discrepant event! T h e evolution of bubbles a t the surface of the Cu ceases! At the conclusion of each reaction, the students are given time to record their observations on a worksheet. Finally the students are directed to explain on the microscopic level their Magnesium connected to copper observations. and immersed in HCi. Hydrogen 4: The three gas evolves at both metals. beakers from the above reactions, w i t h t h e m e t a l strios removed. are disolaved. Concentrated NaOH (12M) is aided to each beaker'to krst neutralize the acid and subsequently to check for the presence of M e ions. The formation of Mg(OHj2 precipitate is observed5 in the beakers from reactions 2 and 3. As expected, no precipitate is observed in the beaker 1. Students also note that the solutions in beakers 1 and 3 are not blue, indicating that Cu ions are not present, which verifies that Cu does not react with the acid. Part B
I n order to aid students in mentally visualizing the electrochemical concepts demonstrated, we developed a computer animation video (4, 5) that models the microscopic events. Students view this animation video and a discussion session follows. Advantages
The Mg sample disappears faster than a Zn sample and this is preferable for a demonstration. Furthermore, using Mg instead of Zn, allows students to see that Hz(gj evolution, a t the surface of the copper, ceases once all of the immersed Mg has reacted. The formation of the Mg(OHl2 visibly emphasizes to the students that the immersed Mg went into solution as ions. The demonstration video enables each student to see each event clearly, repeatedly, or in stop-action mode. The animation video provides students with a visually concrete model of these mieroscopic processes and enables students to improve their own mental models. Conclusion
Magnesium should be used in place of zinc because of its superior reaction speed and the availability of Mg(OH)z a s a visible test of the presence of Mg2'ions. When the discrepant event approach is employed, Mg is again superior to Zn because the audience can observe the cessation of bubbling sooner. Multi-media is the audience's preferred augmentation method6 for visualizing events on both the macroscopic and microscopic levels. Instructors may wish to augment other classical electrochemistry demonstrations (6, 7)by the methods presented here. Volume 72 Number 1 January 1995
55
Acknowledgment We wish to thank the AutoDesk Education Grants Program forHvwrChem.Wu also wish to thank the 1992-1993 Honors en& Chemistry students for their cooperation with and enthusiasm toward this electrochemistry activity. Literature Cited 1. Kolb, D.A.Iaarni"gSLykIxwntory; McleBerb Company: B o s h . MA.1985. 2. Barrel, F L.; Fernandez, E. G. R.; Otem, J. R. G.J. Chem. E d v e 1992.69.655-657.
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3. Petruai, R. H.Ooneml C k m l s t w Principles ond M&m Applicafiona; MaeMillan: New York, 'OO1. 7C' .-. 4. Hype,chhpm R o l ~ - Z . O / o r Windw,ndw,:AutoDesk, hc., ssusalito,WL 5. A n i m f o r P m , Mralan l.O:AutoDesk, h e . . Sausalifo, CA 6. Alyea, 8. N.; Dutton, F B., Eda. %led Dprnonstmlions in Chemistry: Journal of Chemical Education:Easton. PA, 1965:p 17. 7. Sh&ashH. B. 2. Chemical Domomtmfions,Vol. 4: U n i ~ r s i t yof Wisconsin Pleas: Madison. W1, 1992:p 137, No. 11.10.
The Phases of Sulfur Submined by
Kurt R. Birdwhistell Loyola University New Orleans, LA70118 Checked bv
James W.Long
University of Oregon Eugene. OR 97403
Sulfur is a brittle yellow solid in its elemental form. Much of the sulfur currently used in this country is mined from vast deposits found in Louisiana and Texas. Crystalline sulfur exists as sulfur Ss rings or chains. The most common form of sulfur is cyclooctasulfur (Sg) which exists primarily in three crystalline forms (a,P, or y-sulfur) (I). The ability of sulfur to form rings and chains results in a very complex thermal chemistry of elemental sulfur (1). Upon melting (112-119 "C) the Sg rings persist to give a mobile liquid, but further heating results in a n increase in viscosity as the S6 rings cleave to form larger rings and chains. Above 160 "C ring opening polymerization of Sg OCcurs rapidly with a dramatic increase in the viscosity (approximately three orders of magnitude increase in viscosity). At about 200 "C the polymer chains start to break up and the viscosity slowly decreases with increasing temperature until the boilingpoint is reached a t 440 "C. Upon cooling the polymer chains break up and the Sg rings reform because Sg is the most stable form at room temperature. The following demonstration illustrates the dramatic viscositv chanees that SRundergoes uoon heatine to 200 "C and theh cooliGg to room temp&ature. This demonstration
56
Journal of Chemical Education
is a nice sunnlement to the demonstration of the rubberlike propert& of catenasulfur made by rapid cooling of the sulfur melt in ice water. Catenasulfur demonstration is discussed by Shakhashiri (2)or Alyea (3) Materials The first step in assembling the apparatus is to obtain glass tubing approximately 25-30 cm long with 1-2 cm OD. Seal tightly one end of the glass tubing using a torch. After cooling fill the tube half full with elemental sulfur. Seal the other end of the tube securely with a torch. Be sure to use sufficient glass on both ends of the tube to make a strong seal. After sealing the tube should be 20-23 cm long. Other materials needed are a hot plate. (I used a Corning PC320 stimnghot plate.) and test tube holder or hot mit. Caution: Demonstrators and observers should wear safety
glasses. Procedure This demonstration is most effective in a small classroom or lab situation with, at most, 20 students. Turn on your hot plate to approximately 60% power five minutes before the demonstration to allow sufficient time for the hot plate to come to an appropriate operating temperature. Place the sealed glass tube on the hot plate with one end of the tube hanging off to use as a handle. Use the test tube holder to rotate the tube slowly along its long axis. Within a minute most of the sulfur will have melted. Pick up the tube and rock it eentlv back and forth to illustrate the low viscosity of the &furat this point. Place the tube back on the hot d a t e for 1-2 min. Havine the hot h late on a slieht incline (just a few degrees) helps-to keep the sulfur on one end of the tube. After 1-2 min remove the tube from the hot plate and rock the tube gently back and forth to illustrate the high viscosity (virtually no movement) of the sulfur a t this point. Keep rocking the tube back and forth allowing t h e tube to cool, suddenly t h e viscosity will dramatically drop to the viscosity observed earlier. This is an amazing and obvious change. Further cooling will result in the formation of large crystals of sulfur. The whole process can be repeated immediately if desired. I have reused my apparatus a half dozen times. The sulfur tends to take on a dirty brown color after repeated uses. Literature Cited 1. la1 Meyer. B. Cham. Rev. 1978, 76,367. 61 Cotton. F. & Wi&inson, G.Aduonced Inorg.nlc Chemistry: 5th 4. Wiley: New York, 1986,pp49P496. 2. Shafiashin. B Chemical Mmonsrmtions a Handbook for % c h m of Chpmlstry; Univ. ofWiaconain: Madison, WI,1983,Vol.1, p 243. 3. Alyea,H.J. Chpm.Educ 1968,45,A929-A930.
