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MSJ2e_Ch13_ISM1_June26_p565 - Chapter 13 Chemical Kinetics...

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Chapter 13: Chemical Kinetics: Rates of Reactions Chapter 13: Chemical Kinetics: Rates of Reactions Teaching for Conceptual Understanding Kinetics is a topic with which students have many misconceptions. Among them are: (1) a catalyst gives the reactants energy causing the reaction to go faster, (2) a catalyst causes the curve in an energy profile (Figure 13.14) to get narrower or shift to the left, and (3) rate laws have only integer powers. Students usually do not think about kinetics at a particulate level. It is important for them to visualize changing concentrations, orientation, surface area, and speed of reactants and the effect on the rate. If Chapters 13 and 14 are tested together, students will confuse kinetics and equilibrium by (1) confusing k, the rate constant for a kinetic process, and K, the equilibrium constant for an equilibrium process, (2) thinking a small value for k indicates that the reaction will go slowly, and (3) using coefficients from balanced chemical equations for the powers in a rate equation. Suggestions for Effective Learning There are two good demonstrations for the effect of concentration on a rate of a reaction. The first involves: (1) igniting a balloon filled with hydrogen, (2) a balloon filled with hydrogen and oxygen in a stoichiometric ratio, and (3) a balloon filled with hydrogen and a limiting amount of oxygen. The second demonstration involves the reaction of H 2 O 2 , KI, and a small amount of bubble bath solution. Place a cap of bubble bath solution in each of three 1-L graduated cylinders, add 30-50 mL of 3 % H 2 O 2 to one cylinder, 10 % H 2 O 2 to another and 30 % H 2 O 2 to the third, finally add a few crystals of KI to each cylinder. It is wise to place the 30 % H 2 O 2 cylinder in a basin because the foam produced will overflow the cylinder. A dust explosion is a good example of the effect of surface area on the rate of a reaction. The effect of temperature on the rate of a reaction is seen in cooking and in the difficulty in starting a car engine on a cold winter morning. The effect of orientation on the rate of a reaction can be illustrated with felt board and a Ping-Pong ball to which a Velcro circle (the rough half) has been glued. Throw the ball at the felt board, it will stick only when the orientation is correct. A manufacturing assembly line is a good analogy for a rate-determining step in a reaction. The final product can be produced only as fast as any one given step in the many assembly steps. Cooperative Learning Activities Questions, problems, and topics that can be used for Cooperative Learning Exercises and other group work are: Have students role play the effect of changing concentration, orientation of molecules, temperature, and pressure on a reaction’s rate. Questions for Review and Thought from the end of this chapter: 119-123 Conceptual Challenge Problems: CP13.B, CP13.C, and CP13.D Concept map terms: activated complex, activation energy, bimolecular reaction, catalyst, chemical
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