applications and analogies
edited by
RONDELORENZO Middle Georgia College Cochran, GA31014
Pictorial Analogies XII: Stoichiometric Calculations
Excess Reagent
John J. Fortman Wright State University Dayton, OH 45435 It must he pointed out tn students that the answer to the auestion "Which is the meatest nmount?" mav hc different depending on whether ;amount" refers to couit, volume, or mass. Figure 1compares a dozen eggs, six watermelons, and three gold bars. The eggs are the most in terms of number, the watermelons in terms of volume, and the gold in terms of mass. This allows the instruetor to focus the attentions of the students on the fact that in stoichiometric problems i t is the number, or moles, of molecules of a chemical that really matters even though it must he measured out in terms of masses or volumes. (See paper N in this series concerning atomic weights and mass to count relationships.)
Figure 2. An analogy to the stoichiometry of a balanced chemical equation and the result of excess reagent.
.Which is the Greatest Amount?
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Figure 1. An analogy to comparing the number, volume, or mass of samples of differentchemicals. Once this is done, stoichiometricratios in terms of items (or moles of items) can be related to balanced reaction equations. Other analogies to stoichiometricrelationships include fruit baskets ( I ) and dancing couples (2). The stoichiometricrelationship depicted in Figure 2 uses a ratio of four wheels to one body in making wagons. This then is used to illustrate that if one has too many of one part, then that part will be in excess which is an analogy to limiting and excess reagent problems. Other analogies to limiting reagents have been published ( 3 , 4 ) . One particular type of reactant often gives particular difficulty. Those are diatomic elements (5) involved i n stoichiometry problems. A useful analogy, illustrated in Figure 3, is that "Hostess" or other brands of packaged cupcakes cannot be purchased singly, but must be bought in pairs. So also must diatomic elements be used in pairs. Part of presentations at tne F hn B~ennal Conferenceon Chem~cal Eoucal on, Fort Co Ins. CO Ju y 24. 1978. pp €-7 of the ProceeoConferenceon Cnemlcal Edxat8on. ngs. and !he N ntn B~enn~al Bozeman, MT, July 26,1986
Figure 3. An analogy to diatomic elements in chemical reactions.
Stoichiometric Approximation
Figure 4. An analogy to stoichiometrically insignificantchanges, Still another analogy is useful when one considers stoichiometricapproximations in equilibrium calculations. If the amount reacted is very small compared to the total Volume 71 Number 7 July 1994
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starting amount, the loss can be ignored, just like the loss of several lumps of wal from a whole truckload as pictured in Figure 4. This same coal truck analogy also is useful in explaining why a positive ion is given the same molecular mass as its parent atom or molecule in calculations. For negative ions one can ask the student if a fly landing on your shoulder while you are weighing yourself will change your measured weight (6). Full page photocopies of these analogical pictures will be sent on request.
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Journal of Chemical Education
Acknowledgment
Bruce Stiver of Media Services at Wright State prepared these drawings with funding from the chemistry department. Literature Cited 1. Bleam, W,Jn J Chom. Educ. 1881,58,1&4-1g1. 2. Last. A.M.J Chom. Educ. 1983.. M.. 748-750. 3. McMinn, D.J. C k m . Educ. 1984,51,591 4. Sdwramrrh.t P J (hem E0.c 19g1.62.61. 5 Patman. J J Chem 13Nel.v Seprcmbcr 10-8. No. 97, p I R 6 F*flman..l J J C l e m M w . 1982 59.3W-324
