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Unformatted text preview: Chapter 18: Thermodynamics: Directionality of Chemical Reactions Chapter 18: Thermodynamics: Directionality of Chemical Reactions Teaching for Conceptual Understanding The focus of Chapter 18 is the direction of chemical reactions. Use the terms product-favored reaction and reactant-favored reaction instead of spontaneous reaction and nonspontaneous reaction. There is an important distinction: under some particular conditions, any reactant will go spontaneously toward products, even though some reactions will produce very few products before reaching equilibrium. Remind the students that product-favored and reactant-favored relate to the position of equilibrium of reversible reactions discussed in Chapter 14. Students usually grasp the concept of energy and entropy when described in the context of their bedrooms or dorm rooms. A students room (the system) has a tendency to go towards higher entropy during major exam times. The order of the system is restored only after energy is expended while cleaning the room. When discussing the qualitative rules for entropy, present examples on both a macroscopic level and a symbolic level. Students who may understand the theory behind the rules may not make the connection when interpreting the reaction states of reactants and products in a chemical equation. Students are easily confused by the terms H , G and S. It is important to discuss the similarities and differences among them. Suggestions for Effective Learning As mentioned in Chapter 4, use the terms reactants and products instead of the terms left-side and right-side. Some students with learning disabilities are easily confused by these latter descriptions. Students sometimes make simple errors when doing thermodynamic calculations because they overlook the fact that the units of H , and G are kJ/mole while the units of S are J/mol. Point out this fact to your students and remind them that, when adding or subtracting terms, such as in the equation: G = H T S, the units must be the same. If a large percentage of your students are pre-professional health career majors, it is very important to cover applications of Gibbs free energy to biological systems. Many biology courses assume students have learned this material in depth in a chemistry course. Remember the terms exothermic and endothermic are not used in life science courses, instead exergonic and endergonic are used. Cooperative Learning Activities Have students think of everyday examples of high and low entropy. Conceptual Challenge Problems: CP-18.B (have students flip coins to verify the outcome) and CP- 18.D. Questions for Review and Thought: 103, 104, 111,112, and 122....
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