In the Classroom
applications and analogies
Heat Flow vs. Cash Flow: A Banking Analogy1 Charles M. Wynn, Sr. Eastern Connecticut State University, Willimantic, CT 06226-2295 Prior knowledge can have a significant impact on subsequent learning (1). Such knowledge utilized in analogies can aid students in bridging the gap from an understanding of familiar phenomena to an understanding of more abstract ones (2). One excellent example is prior experience with automated teller machines (ATMs), which can provide insights into the underlying nature of exothermic and endothermic reactions. Consider a customer who needs $200 to purchase a new radio. The customer, who has $1000 in his or her account, withdraws the necessary cash from an ATM. This “exocashic” action converts 200 of the potential dollars in the account into 200 actual dollars (cash) and thereby reduces the potential dollars in the account to 800. Money flows out of the ATM system and into the surroundings, the customer’s wallet. The potential $ of the system decreases while the actual $ of the surroundings increases. Such financial transactions can be represented in various readily understandable ways as shown in Figure 1. Now consider the reaction: C(s) + O2 (g) → CO 2(g) + 393.5 kJ (Fig. 2). This exothermic reaction converts 393.5
kJ of the chemical potential energy of the reactants into 393.5 kJ of actual heat (thermal energy). The chemical potential energy of the products is therefore 393.5 kJ less than that of the reactants. Heat flows out of the chemical system and into the surroundings—for example, a bomb calorimeter. The chemical potential energy of the system must decrease while the actual heat of the surroundings increases. This analogy provides insight into the source of heat that emerges in an exothermic reaction. Analyses of “endocashic” and endothermic reactions follow similar reasoning processes and provided similar insights. Note 1. Presented at the 13th International Conference on Chemical Education, Inter-American University of Puerto Rico, August 9, 1994.
Literature Cited 1. Shulman, L. AAHE Bull. 1989, June, 8–13. 2. Piaget, J. J. Res. Sci. Teach. 1964, 2, 176–186.
393.5 kJ
cash flow = ∆ $ = $ content final – $ contentinitial = $800 – $1000
= -393.5 kJ ∆H is negative; ∆H < 0.
➤
= { $200 ∆ $ is negative; ∆ $ < 0.
heat flow = ∆ H = heat contentproducts – heat contentreactants
$ content (stored in bank account)
Figure 1. “Exocashic” action.
Figure 2. Exothermic reaction.
Vol. 74 No. 4 April 1997 • Journal of Chemical Education
397
