Lecture 07_Ch15c - Chapter#15 Chemical Kinetics 15.1...

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15.1) Reaction Rates 15.2) Rate Laws: Introduction 15.3) Determining the Form of the Rate Law 15.4) Integrated Rate Law 15.5) Rate Laws: Summary 15.6) Reaction Mechanisms 15.7) The Steady-State Approximation 15.8) A Model for Chemical Kinetics 15.9) Catalysis Chapter #15 – Chemical Kinetics
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A Model for Chemical Kinetics (Zumdahl Section 15.8) Concentrations of reactants affect rates So does temperature So do catalysts Obviously, a rate constant is not really a universal constant , but is constant only for a fixed set of experimental conditions . How do we explain the dependence of reaction rates on temperatures and catalysts? i.e. Where do rate constants come from?
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Collision Theory of Reactions Consider a gas phase reaction: 2A(g) products This reaction requires collision of 2 reactants. We can calculate the collision frequency from the ideal gas law. (Zumdahl, Section 5.9) We can measure the reaction rate experimentally. For such reactions, the reaction rate is typically several orders of magnitude slower than that predicted from the collisional frequency alone. We can conclude that not every collision of reactants results in a successful reaction .
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What factors might keep the two reactants from reacting once they have collided? One obvious candidate is molecular orientation : only the correct orientation of reactants will lead to successful product formation This could account for perhaps 1 order of magnitude, but not for the several orders of magnitude reduction in rates observed experimentally. The answer must be more complex. Figure15.1 1
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Arrhenius believed that for molecules to react upon collision, they must become "activated," and the parameter Ea has become known as the Activation Energy. Only some collisions occur with enough energy to overcome the activation barrier (Ea). Figure 15.9
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E tells you if the rxn. is exothermic or endothermic, but is not related to rate and doesn’t tell you anything about the rate. Ea for the reverse reaction can be calculated using Ea for the forward reaction and E. E
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Lecture 07_Ch15c - Chapter#15 Chemical Kinetics 15.1...

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