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Kinetic_ans

# Kinetic_ans - Revised Spring 2000 Kinetics Supplementary...

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Revised Spring, 2000 Kinetics Supplementary Sheet 1. From the following reaction mechanism determine the overall chemical equation and the rate law. 2B ----> B 2 (fast) A + B 2 ------> AB 2 (slow) AB 2 -------> B + C (fast) The overall equation for this reaction is simply the sum of the individual steps in the reaction. In this case the equations add up to the equation A + B -------> C . The rate law is derived from the slow step in the mechanism, for this reaction the rate law is rate = k[A][B 2 ]. However, rate laws are usually given in terms of the reactants not an intermediate. Since the first step of the reaction involves the formation of B 2 from 2B in a fast step, the [B 2 ] is proportional the [B] 2 . The rate law then becomes rate = k[A][B] 2 . 2. For the reaction A + B 2 -----> AB + B, which is catalyzed by C, the rate law is rate = k[B 2 ][C]. Suggest a mechanism, indicating the slow step, for this reaction that agrees with this rate law. Since B 2 and C are involved in the mechanism, they must combine in the slow step to form some type of intermediate, such as B 2 C. This intermediate then combines with A in a fast step to produce the products AB and B along with the catalyst C. This shows how C can be neither a reactant nor a product yet still be present in the rate law. B 2 + C -------> B 2 C (slow) B 2 C + A -------> AB + B + C (fast) B 2 + A -------> AB + B (desired overall equation) Note: Other mechanisms are also possible, all involve the reaction of C in or before the slow step and the reformation of C later as a product.

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3. The following kinetic data were obtained for the reaction: 3A + 2B -------> 2C + D.
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Kinetic_ans - Revised Spring 2000 Kinetics Supplementary...

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