A "Nuts and Bolts" Approach to Explain Limiting Reagents An oftentimes perplexingprohlem for the student beginning the study of chemistry is the concept of limitingreagents. An initial obstacle is recognition that themasses of the reagents do not directly determine which is the limitingreagent; one must think of the combination of reagents in terms of unit collections of molecules: moles, not grams. The second hurdle comes when a reaction stoiehiometry is not the simplest case of 1:l. I have used some analogue "nuts and holts reactions" to address these points, these "reactions" should prove useful in the high school or remedial college class setting. These analogies can be directly related to real chemical reactions. Standard nuts and bolts are obtained from a hardware store; the only requirement for these props is that they are large enough to he clearly distinguishable to the class. The first example with 1:l stoichiametryis the Lewis acid-base reaction of boron trifluoride with ammonia: BF3 NHa + H3N-BF3. One holt represents a unit of BF3, a molecule or a mole. Likewise, a nut represents the lighter ammonia. Amounts of the two "reactants" are weighed into separate beakers: 272g of bolts ( B F 3 and 150g of nuts (NH3).' These amounts are weighed in front of the students, which allows them to see the actual masses of the reactants for veracity. Preferably, an electronic balance with a digital display is used. This process mare closely parallels what is performed in the lab; one cannot measure moles on a "mole balance", quantities such as mass, volume, or pressure are measured and translated into mole amounts. The model chemical reaction is written on the blackboard and the gram amounts are shown beneath the respective r e a ~ t a n tThe . ~ "reaction" is run by pouring the nuts into the beaker of bolts and mixing. One nut and one bolt are removed and their chemical union is shown by threading the two together, which neatly indicates adduct formation, This "product" is placed into another beaker and one continues with the reaction. The supply of bolts is depleted before that of the nuts despite the fact that there was a greater mass of bolts. The holts, therefore, are the limiting reagent. One then proceeds to calculate the mole amounts of BF3 and NH8 for the real reaction, based upon the masses above, and discusses the reaction staichiometry with reference to the nuts-andholts analoev. -. Another examole reaction can serve for the case where the reaction stoiehinmetrv is aimnlv Two he,~ ~ ~ ~ not . ~ ~ ~ . . 1:l. ...~ . ~ .~ ~ nnta ~ ~ can r ~ "bonded" together with some wire. Each nut represents a CI atom: the aasemhlage is molecular chlorine. The model reaction is 2C3 T Clr 2C'sCl. where a holt represents the heavier Cs. A "reaction mixture" consists of 80 g (1.1 mull uf "CI?" and 204 g (1.5 mol) of"Cs3'. The "hond" m CI? is broken and each CI a t m is attached toa Cs. I)rspice the fact that cesium is present in greater mole and mass amounts, it is consumed while some C12still remains. Mathematical methods for determining limiting reagents, such as that recently described by Kalantar? can he introduced with this example and the agreement between theory and "experiment" can be verified.
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'Average weights of W nutsand Dolts wed here were 10 and 349. respectively. Actual welghts of me 'reactants.' varied by + 3 g hom "reaction' to 'reaction" due to me variabilily in mass at Indiviaa nuts and oom. Good bwkkeeping practices should be re-emphaslzed with mese stolchlomeby problems by writing the gram amounts. mole amoums, and formula weights beneaIh each reactant in t h e balanced chemical equation. Kalantar, A. H. J. Chem Muc. 1985, 62, 106. Cralg Blankanshlp Carleton College Nmlhtield. MN 55057
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Journal of Chemical Education
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