February 13th 2017 Ch 13 HW due tonight Ch 16 HW Keep up with Ch 16 16 3 The Rate Law and its Components For any general reaction occurring at a fixed temperature aA bB cC dD Rate k A m B n The term k is the rate constant which is specific for a given reaction at a given temperature The exponents m and n are reaction orders determined by experiment Note The values of m and n are not necessarily related in any way to the coefficients a and b Determining Reaction Orders A reaction has an individual order with respect to or in each reactant For the simple reaction A products First order rate A Rate k A 1 k A First order with respect to A Second order rate A 2 Rate k A 2 Second order w r t A Zero order rate not dependent on A Rate k A 0 k 1 k Zero order w r t A rate k A m Individual and Overall Reaction Orders For the reaction 2NO g 2H2 g N2 g 2H2O g Rate k NO 2 H2 The reaction is second order with respect to NO first order with respect to H2 and third order overall 2 1 3 For the reaction CHCl3 g Cl2 g CCl4 g HCl g Rate k CHCl3 Cl2 1 2 The reaction is first order with respect to CHCl3 order with respect to Cl2 and 3 2 order overall 1 3 2 Reaction orders must be determined from experimental data and cannot be deduced from the balanced equation Determining Rxn Orders by Changing Reactant Concentrations Initial Rate Experiment mol L s Initial A mol L Initial B mol L 1 1 75x10 3 2 50x10 2 3 00x10 2 2 3 50x10 3 5 00x10 2 3 00x10 2 3 3 50x10 3 2 50x10 2 6 00x10 2 4 7 00x10 3 5 00x10 2 6 00x10 2 Rate k A m B n Rate k A m B n Finding m the order with respect to A We compare experiments 1 and 2 where B is kept constant but A doubles Rate 2 Rate 1 k A m B n 2 k A m 1 2 B n1 m A 2 m A 1 A 2 A 1 m 3 50x10 3 mol L s 1 75x10 3mol L s 5 00x10 2 mol L 2 50x10 2 mol L 2 00 2 00 m log 2 00 m log 2 00 m 1 m Rate k A m B n Finding n the order with respect to B We compare experiments 1 and 3 where A is kept constant but B doubles Rate 3 Rate 1 k A m B n 3 k A m 1 3 B n1 n B 3 n B 1 B 3 B 1 n 3 50x10 3 mol L s 1 75x10 3mol L s 6 00x10 2 mol L 3 00x10 2 mol L 2 00 2 00 n n 1 Rate k A B n Sample Problem 16 3 Determining Reaction Orders from Rate Data Many gaseous reactions occur in a car engine and exhaust system One of these is NO2 g CO g NO g CO2 g rate k NO2 m CO n Use the following data to determine the individual and overall reaction orders Experiment Initial Rate mol L s Initial NO2 Initial CO mol L mol L 1 0 0050 0 10 0 10 2 0 080 0 40 0 10 3 0 0050 0 10 0 20 NO2 experiments 1 and 2 CO experiments 1 and 3 rate k NO2 m CO n To calculate m the order with respect to NO2 we compare experiments 1 and 2 rate 2 rate 1 k NO2 m2 CO n2 k NO2 m1 CO n1 NO2 2 m 0 080 M s 0 0050 M s NO2 1 m 0 40 M 0 10 M 16 4 0 m The reaction is second order in NO2 m 2 To calculate n the order with respect to CO we compare experiments 1 and 3 rate 3 rate 1 k NO2 m3 CO n3 k NO2 m1 CO n1 CO 3 CO 1 n 0 0050 M s 0 0050 M s 0 20 M n 0 10 M 1 0 2 0 n The reaction is zero order in CO n 0 rate k NO2 2 CO 0 or rate k NO2 2 second order overall Experiment Initial Rate mol L 1 s 1 Initial NO2 mol L Initial CO mol L 1 0 0050 0 10 0 10 2 0 080 0 40 0 10 3 0 0050 0 10 0 20 NO2 4x CO 2x NO2 rate 16x rate COrate 1x rate 4 m 16 m 2 2 n 1 n 0 Experiment S2O82 initial M I initial M Rate M s 1 0 038 0 060 1 4 x 10 5 2 0 076 0 060 2 8 x 10 5 3 0 076 0 030 1 4 x 10 5 What is the rate order for S2O82 a b c d e 0 1 2 3 4 rate 2 rate 1 2 8 x 10 5 M s 1 4 x 10 5 M s k S2O82 m2 I n2 k S2O82 m1 I n1 0 076 M m 0 038 M 2 2 m m 1 The order of S2O82 is 1st order S2O82 2 S2O82 1 m Rate k A m B n Determining the Rate Constant specific for a particular reaction at a particular temperature units depend on the overall order Overall Reaction Order Units of k t in seconds 0 mol L s or mol L 1 s 1 1 1 s or s 1 2 L mol s or L mol 1 s 1 3 L2 mol2 s or L2 mol 2 s 1 Given the following data determine the value of the rate constant for the following reaction S2O82 3I 2SO42 I3 rate k S2O82 I Experiment S2O82 initial M I initial M Rate M s 1 0 038 0 060 1 4 x 10 5 2 0 076 0 060 2 8 x 10 5 3 0 076 0 030 1 4 x 10 5 rate k S2O82 I a 0 038 L mol 1 s 1 b 0 060 L mol 1 s 1 c 0 00228 L mol 1 s 1 d 1 4 x 10 5 L mol 1 s 1 1 4 x 10 5 M s k 0 038 M 0 060 M k 1 4 x 10 5 M s 0 00228 M2 e 0 0061 L mol 1 s 1 k 0 0061 L mol 1 s 1