applications anJ analogies Analogical Demonstrations John J. Fortman Wright State University Dayton, OH 45435
Many students up to and including those in first-year college courses are oRen still making the transition from concrete ooerational reasonine (dealine with observables) to formal operational reasoning (dealing with the abstract). Manv concepts of chemistnr involve abstract models or theoriks. As such, qualitati;e analogies of concrete observable items to abstract concepts may be helpful both in their understanding of the concept and in hastening their development from the concrete to the abstract reasoning level: Visual aids and demonstrations give a student something to which to connect a concept or fact, and, as such, help the student to remember and-recall that information later. By illustrating a n analogy with a visual demonstration, a student is aided in both understanding and remembering a lesson. The use of stair steps to represent quantized energy levels is a classic example of this.
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Chemical Combination versus Physical Mixtures Using some type of interlocking blocks1 of two different colors, contrast a container ofunconnected blocks (physical mixture of variable proportions) and a container of pairs of the two different color blocks snapped together. FisherPrice Snap-Lock Beads2 work very well for this purpose and for other demonstrations requiring interlocking blocks. The physical mixture can be separated by sorting by color, but to separate the parts in the pairs requires "breaking a bond." Conservation of Mass in Chemical Reactions (2.3 . . . Take a mixture of two different colored blocks. (There need not be the same number ofeach., Snap them together into identical patn. (If thcre are extras of one color simply put them with the a)mpleted pairs.) Ask the class if the final total weieht is anv different from the startine total weight. By using two to one or other combinations this also illustrates stoichiometric ratios and excess and limitine rea g e n t ~Simple .~ toy blocks have the advantage over milecular model kits in that thev are more familiar to the students. Not beingUspecialshentific tools" they are easier to relate to and remember.
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Presenlm at the 198th Nat onal Meeting ofthe American Chemica Socety. M~am Beach. FL. Sepl. 14. 1989. 'Legos have also been used ( 1 ) . 2Fischer-PriceSnap-Lock Beads can be purchased in the toys for toddlers section of most department stores. The cost may range from $5 to $10. 30ther stoichiometric and limiting/excess reagent analogies have been suggested using fruit baskets (4,bicycles (5),dancing couples (6), and coins for coffee (7). 40theranalogies suggested forpolymers are freighttrains (9),and chains of students (10). 'Attribute Blocks can be purchased from Creative Publications. Order Department, 5005 West 110th Street, Oak Lawn, IL, 60453 (cat. no. 30401) for $22. 323
Journal of Chemical Education
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Law of Multiple Proportions (2,3) Snap toeether beads1 of different colors in a 1:l and a 2:l ratio. -Askss;ming identical masses for the same color beads discuss the conseauences. There is a simple whole number ratio (12)of the masses of the first colorbeads per identical mass of the second. The Law of Definite Proportions can be discussed in a similar fashion. Structural Isomers (2,3) . Using Snap-Lock beads1 make 2 1 combinations of two different color beads i~ BAB and ABB sequences. Both have the same 2:l empirical formulas, but different structures. As with the illustration of the conservation of mass the beads are more familiar than model kits for the bepinning student and also connects their thoughts to the s&ichiometric ratio demonstrations. ~
Masses of Ions and Atoms Use a marking pen to make a mark on a block to represent a negative ion with its extra electron. Ask the class if the weight difference is significant. Alternatively, ask the class if a fly landing on you while you are being weighted changes your measured weight. Polymers and Copolymers (6j4 Link together a string of identical Snap-Lock beads that represents a polymer of identical monomers. Make a string by randomly combining two different color beads to illustrate a copolymer. Note that each bead now represents a monomer instead of an atom. Amino Acid Sequences Note that each different color bead now represents a different amino acid. Snap-lock beads have a male and a female end (or inner and outer ioints). so AB can be shown to be different from BA. Ask Low many different combinations of three can be made from one each of three different colors. Show them the n! = 6 different sequences. Ask how manv three-member seouences can be made if each color can be used up to three'times in each trimer. Show them the n" = 27 different seauences. (This rewires 33 of each of 3 colors or 11 sets of snaplock beads.) it is good to make these available for the students to examine in more detail in the laboratory Classification by Properties and Attribute Blocks Attribute Blocks5 (Invicta Plastics Ltd.) consist of 60 pieces in all combinations of three colors, five shapes, two sizes, and two thicknesses. The blocks can be classified by these four properties and it is seen that the classes overlap one another. It is shown that a maximum of only eight yesno questtons about properties need to be asked inorder to identify a specific block out of the 60 possibilittes. The value of generalization and classificattnn is illutrated. Heat Transfer ( 8 ) If something gets hot by gaining heat why doesn't it get cold by gaining cold? Have two glasses, one full of water and one empty. Pour some water6 from the full to the empty glass. The empty glass gained fullness (hotness) by
gaining water (heat) while the full glass gained emptyness (cold) by losing water (heat). Alternatively ask the students how someone gets rich. (They gain money.') How do they get poor? (They lose money.) Something gets hot by gaining heat and cold by losing heat. One must be careful that students do not carry this analogy too far, as energy is consenred but wealth is not. In both cases it is important to remind the students that unlike water or money, heat is not a substance. Conjugate Acid-Base Pairs (2,14) Pass a volleyhall or large bcach ball back and forth with a student. Something that gives ur,a proton has the ahilits proton has the abilto gain it back. something that ity to give it up again. Relative strength can be related to the forcefullness with which the ball is given up or held. Note that in solution a proton is never free from a solvent molecule, so perhaps it is better to hand the ball back and forth. This analogy of proton transfer can be tied into the Lewis definitions. Why can something accept a beach ball (proton)? Because it has a pair of hands (electrons)!
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Ionic versus Covalent Bonding Take out a dollar and have a student take out a dollar. Take the student's dollar and hold the pair. The student is attracted to the dollars, as a positive ion is attracted to the excess electrons of a negative ion. The teacher and the student each holds one end of each of the two dollars. The dollars are shared as in covalent bonding. Pull the dollars closer to you and there is polar covalent bonding. Have one person put up both dollar bills for sharing and coordinate wvalent bonding is illustrated. Orbits versus Orbitals If one wishes to illustrate a ~roblemwith the Bohr d a n etary model, do the fi)llowing.k t ~ a c ha light stick to a birce of strine. Dim the lirrhts and swine the lieht stick around in a circle. Let the string wind around your finger and you illustrate it spiralling into your hand as an electron would spiral into the nucleus. Cover the light stick and move it about in the dark showing a periodic flash. Show more exposures about some given point. Discuss the relative probability of finding the
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electron a t given distances and directions about that point. Note that nothing can be said about the pathway.' Orbital ~ ~ b r i d i z a t i o n ~ Take four glasses of the same size. Fill three of them with one type ofjuice or soft drink and the other one with a juice or soft drink of a contrasting color. Pour all four into a "wave mechanical" pitcher which represents the mixing of atomic wave functions. Pour out four equal glasses of the mixed fluids. Each glass (hybrid orbital) now contains the same amount ofjuice (electron probability),but it has part of each starting glass (unhybridized orbital). Students take more notice if you use disgusting combinations of juices like prune and sauerkraut. Literature Cited ~~~
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1. Deeae, W. C.Preaentedatthe 198thNationslACS Mesting,MiamiBeach, FL, Sept. 12.1989; Abetret No. C H E B S . 2. F ~ m n a nJ. , J. Presented at the 5th B i d Conferwce on Chemicnl Eduestio", Fort Collins, CO, July 24, 1 9 7 8 ; P d i n g s . pp 6 7 , 3. Fortman, J. J. Resented at the 9th Biennid Confm"c8 rm Chemical Education, Bazernan, MT July 26,1986; Abetract No. 1514. 4. Bleam, W. Jr, J Cham. Edue. 1881,58,18&5 6. Silversmith. E. E.J. Chem Edue I%%..62..61. 6. LaaZ A. M.,> c&. Educ 1989.60, 748-750 7. M c h , 0..J. Chem Educ. 1984,61,591 8. Fortman, J. J. Resented at the 195th National ACS Meeting and 3rd C h e d crngess of NmthAme"ca,lbmnta, Canad*, Jvne 9,1988. 9. Hoyt, W. C. J. Chem. Educ 198%65,718 10. Sarquis,A.M. J Chrm. Edue. 1988.63,60-61 11. White, A. D.J Cham. Edue 1881,56,645 12. ten Hoar, M J;Jacobsseholengememchhp,A. J. Chem Edue. 1064, 60,132. 13. VsnLubeek,H.J. Cham. Edur. 1983.60,1983. 14. Fortman, J. J. Cham 13 News 1978, (97). 18. 15. Bard. J.R. J.Chsm. Educ 1981.58.480.
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Peekham.G. D.J. Chem Edm. 1984.61,888
%ater transfer has also been suggested as an analogy to oxidation-reduction processes (11, 12). 'Money transfer has been used as an analogy to proton transferby weak and strong acids (13). Sother analogies suggested for electron motion about atoms are moire' patterns (15) camera piclures of car light densities (16);and blurred pictures (17)A nice analogical demonstration of the Uncertainly Principle involves offset trans-parencies (18, 19) $Thisanalogy was shared with me by someone at the 5th Biennial Conference on Chemical Education at Colorado State University in 1978. but I have since forgotten her name.
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