Chapter%2013%20Chemical%20Kinetics - Chapter 13: Chemical...

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Chapter 13: Chemical Kinetics Chemical Kinetics – The study of rates of chemical rxn – analyzing the intermediates steps between initial to final steps I. Reaction Rate – Change in concentration of one of the reactants or products divided by the time interval A. Average rate of disappearance of R = - Δ [R]/ Δ t (R Final - R Initial ) B. Average rate of formation of P = Δ [P]/ Δ t C. Unique avg. rxn rate Shows the relation between react and prod Coefficients play an important role EX: aA + bB cC + dD Unique avg. rxn rate = -1 Δ     [A]     = -1 Δ     [B]     = 1 Δ     [C]     = 1 Δ     [D]     a Δ t b Δ b c Δ t d Δ t Ex1: 2H 2 O 2 2H 2 O + O 2 in the interval of 60 sec, the [H 2 O 2 ] decreased from 0.88M to 0.697M. What is the unique avg. rxn rate for all of the species involved in the rxn? Unique rxn rate = -1 [0.697M – 0.88M] = -1 [-0.183M] = 0.001542 M/s 2 60s 2 60s Ex2: The average rate of the reaction N 2 (g) + 3H 2 (g) 2NH 3 (g) over a certain period is reported as 1.15(mmol NH 3 ) L -1 h -1 . a) What is the avg. rate over the same period in terms of the disappearance of H 2 ? b) What is the unique avg. rate? a) 1.15(mmol NH 3 ) L -1 h -1 x 3 mmol H 2 _ = 1.73 (mmol H 2 ) L -1 h -1 2 mmol NH 3 b) 1.15(mmol NH 3 ) L -1 h -1 = 0.575 mmol L -1 h -1 3 II. Instantaneous Rate of Reaction - Slope of a tangent drawn to the graph concentration as a function of time - The rate decreases as the reaction proceeds III. Rate Laws - An expression for the instantaneous rxn rate in terms of the [] of a species at any time - Expression can contain react, prod, catalyst, etc., but usually only starting materials A. Rate constant – k (M/s) Each rxn has its own constant k B. Rate = constant x [concentration] a Rxn order is not related to coefficients Ex: 2N 2 O 5 4NO 2 + O 2 Rate Law = k [N 2 O 5 ] 1
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C. First-order Law Rate = k [A] 1 If A is doubled (R = k [2A]), the rxn is 2x fast Units of k : s -1 Formula: ln[A] = - k t + ln[A] o Plot ln[A] vs. time is a straight line Slope = -k (negative slope) D. Second-order Law Rate = k [A] 2 or Rate = k [A][B]
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Chapter%2013%20Chemical%20Kinetics - Chapter 13: Chemical...

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