Chapter-13-Tro

Chapter-13-Tro - Chapter 13: Chemical Kinetics In this...

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Chapter 13: Chemical Kinetics In this chapter we will learn about :(a). rates at which reactions proceed as the reactants are converted to products and (b). reactions mechanisms. Mechanisms of reactions are the detailed pathways by which reactants combine ( for example, do collisions take place between two reactants or more? Are the intermediate species formed in a step involved in subsequent reactions? Rates of Chemical reactions : The rate of chemical reaction is determined by measuring the changes in concentration of a species as a function of time . For example, in monitoring a reactant, what is the decrease in concentration of that reactant in a given time interval?; in monitoring a product, what is the change in concentration of that product in a given time interval? The rate of a reaction can depend on (a). concentration of reactant (as molecules must collide to react); (b). Physical state (frequency of collisions depends on the physical state); (c). temperature (energy of collision increases with temperature; and (d). The use of a catalyst in the reaction (a catalyst accelerates the reaction without being consumed in the reaction). This reaction rate, which has units of mol.L -1 .s -1 depends on which time interval was used for measurement, because the rate of change can depend on time. That is, rate can be higher in the initial stages and lower in the later stages. reaction rate = (change in concentration/change in time).
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Consider the reaction, C 2 H 4 (g)+O 3 (g)=C 2 H 4 O(g)+O 2 (g). For every molecule of C 2 H 4 (g) one molecule of O 3 (g) must react in order for the products to form. Then for every molecule of the product one molecule of the reactant must have been consumed. The rate of decrease in concentration of C 2 H 4 (g) should be same as that of O 3 (g) and same as the rate of increase in concentration of C 2 H 4 O(g) or O 2 (g Here the subscripts 1 and 2 represent two different points during the course of the reaction. When the rates are determined from the decreasing concentration of a reactant, a negative sign is placed in the equation for the rate. When the rates are determined from the increasing concentration of a product, a positive sign is placed in the equation for the rate . time(seconds) [O 3 ]in mol.L -1 Average rate mol/(L.s) 0 3.20E-05 10 2.42E-05 7.80E-07 20 1.95E-05 4.70E-07 30 1.63E-05 3.20E-07 40 1.40E-05 2.30E-07 50 1.23E-05 1.70E-07 60 1.10E-05 1.30E-07 Suppose we have a known concentration of O 3 (g) in a closed vessel. After a sufficient amount of C 2 H 4 (g) has been introduced into the vessel, the concentration of O 2 (g) is found to decrease as shown in the table to the side. The average rates calculated for every 10 s interval [ ] [ ] [ ] t H C t t H C H C time in change H C of ion concentrat in change rate - = - - - = - = 4 2 1 2 1 4 2 2 4 2 4 2 [ ] [ ] [ ] t O t t O O time in change O of ion concentrat in change - = - - - = - = 3 1 2 1 3 2 3 3 [ ] [ ] [ ] t O H C t t O H C O H C time in change O H
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This note was uploaded on 03/06/2010 for the course CHEM 102B taught by Professor A during the Spring '09 term at Vanderbilt.

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Chapter-13-Tro - Chapter 13: Chemical Kinetics In this...

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