Ch 15 Kinetics

Ch 15 Kinetics - Chapter 15. Chemical Kinetics Chemical...

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Chapter 15. Chemical Kinetics Chem. 1B W05 VanKoppen Chemical Reactions have two very important characteristics: 1) Position of Equilibrium (thermodynamics) 2) Reaction Rates (kinetics) A spontaneous reaction, G < 0, does not mean the reaction will be fast. Thermodynamics: 1) indicates relative stability of products and reactants and the equilibrium position (the concentration of products relative to the concentration of reactants at equilibrium) 2) independent of path between reactants and products 3) indicates why a reaction occurs G = H – T S Kinetics: 1) study of reaction rates and reaction mechanism (reaction steps) 2) depends on the path between reactants and products => Kinetics tells you how a reaction occurs (the mechanism) and how fast . Consider the change in concentration as a function of time: Initially only reactants are present and only the forward reaction occurs. As products are produced the reverse reaction occurs until equilibrium is reached. See Figure 6.2, 6.3 and 6.4 pg 192-194 text. At equilibrium : the forward rate = the reverse rate . When can we ignore the reverse reaction? 1) When reactants are first put together. In this case, the forward reaction dominates and we can ignore the reverse reaction, 2) When the reaction goes to completion, that is when K >> 1 (many reactions go to completion), we can only measure the forward rate and therefore we can ignore the reverse rate. See Figure 15.1 pg 705 text. This Figure shows that at a given point (time, concentration) the forward rate corresponds to the change in concentration with respect to change in time. For the reaction: A products Rate = – d[A]/dt = k [A] n In this equation, k is the rate constant and n is the order of the reaction with respect to A. The rate constant, k, and the reaction order, n, are determined from experiment. The integrated rate law allows us to determine the concentration of the reactants or products at any time during the course of a reaction. Zero Order
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Ch 15 Kinetics - Chapter 15. Chemical Kinetics Chemical...

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