kinetics_13

the rate constant rate law rate rate kaxby

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Unformatted text preview: Law Expressions Rate Determination of Rate Law Expressions Determination Initial Rates Method Measured at the beginning of the reaction. Series of Measured experiments are run in which the concentration of reactants are changed to study their effect on the speed or rate of the reaction. speed Graph Method Collected data is graphed in different forms to Collected investigate which one will give a straight line. Each reaction order will generate a straight line under different conditions. different Initial Rates Method Initial For the reaction 2 NO(g) + 2 H2(g) → N2(g) + 2 H2O(g) at 1280°C the following data has been collected. Determine the rate law for the reaction and the rate constant. the Ex peri ment [ NO ] [ H 2] I niti al Rate (M/ s) –3 –3 –5 1 5.0 x 10 2.0 x 10 1.25 10 x –3 –3 –5 2 10.0 x 10 2.0 x 10 5.00 x 1 0 –3 –3 –5 3 10.0 x 10 4.0 x 10 10.0 x 10 Initial Rates Method Initial Establish the ratio of one experiment to the Establish other. One substance’s concentration must remain One constant in the first step. constant General Equation: Rate2 Rate Rate1 Rate = = [A]2 [A] [A]1 x [B]2 [B] [B]1 y If [ ] remains constant, cancel it out. Initial Rates Method Initial For the reaction: S2O82-(aq) + 3 I-(aq) → 2 SO42-(aq) + I3-(aq) For use the following data to determine the rate law and rate use constant. constant. Experi ment [I-] I niti al Rate (M/ s) [S2O82 ] 1 2 3 A) =k[S2O8-2] 0.080 0.080 0.16 b) = k[S2O8-2][I-] 0.034 0.017 0.017 c) = k[S2O8-2]2 -4 2.2 x 10 -4 1.1 x 10 -4 2.2 x 10 d) k[S2O8-2]2[I-] Order of Reactions Order Zero Order Does not depend on any reactant First Order Depends on one substance directly Second Order Depends on one substance to the second Depends power. power. Depends on two substances directly Integrated Rate Laws Integrated First Order Reactions Rate = k[A] Rate ∆[ A ] Rate = ∆t ∆[ A ] − = k [ A] ∆t ∆[ A ] − = k∆t [ A] t ∫ 0 ∆[ A ] = ∫ − k∆t [ A] 0 t [A] ln [ A] t = -kt 0 ln[ A]t - ln[ A ]0 = -kt Integrated Rate Laws Integrated First Order Reactions ln[ A] t - ln[ A ] 0 = -kt Equation of a line Equation Y = mx + b Integrated Rate Laws Integrated How long will it take for a first order reactant with original How concentration of 0.157 M to decrease to 0.0565 M? The rate constant is 1.34 x10-3 1/s rate A) 268 s b) 762 s c) 3.72x10-3 s d) 245 s What will be the conc. of A after 12 min? A) 0.0598M b)0.412M c) 0.154 M d) 0.159M Half-life Half-life Time that it will take for the Time [ A] concentration of reactant to ln decrease to ½ of its original 2[ A] value. (t ) value. 0 = -kt1 / 2 0 1/2 0 t1/2 when [A]t = [A]0/2 1/2 when Substituting in the integrated Substituting equation for first order. equation [ A] ln [ A] t 0 = -kt 1 ln = -kt1 / 2 2 − ln 2 = -kt1 / 2 0.693 = kt1 / 2 0.693 = t1 / 2 k Half-Life Half-Life For First Order Reactions: t1/2 = 0.693 0.693 k Important for radioactive substances that are known to Important decay by first order reactions. decay When...
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This note was uploaded on 09/29/2013 for the course CHM 1046 taught by Professor Staff during the Fall '08 term at FIU.

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