17 - Rates

# 17 - Rates - Chemical Kinetics Rate Laws a rate law is an...

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Chemical Kinetics: Rate Laws a rate law is an equation expressing the rate of a reaction in terms of the molar concentrations of the species involved in the reaction rate = k [A] m [B] n In this rate law, m and n are orders of the reaction components k is the rate constant The overall reaction rate has an order that is m + n

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Chemical Kinetics: Determining the Orders of a Chemical Reaction • the method of isolation puts all but one reactant in such a large excess that their concentration change over the course of the reaction is essentially zero ! the order determined for the remaining reactant is a pseudo-order for the reaction ! the pseudo-orders can be determined for each reactant, and can be used to give the overall order • the method of initial rates measures the rate at the beginning of the reaction (<10% conversion) for several initial concentrations of the reactant There are two common experimental methods that help us determine the orders of a reaction:
Using the Initial Rates Method Initial Rates Method assumes that [A] 0 isn’t changing during course of measurements Example : the initial rate of a reaction depended on the concentration of a substance A as follows: 130 41 9.6 3.6 rate 0 (10 -7 M s -1 ) 30 17 8.2 5.0 [A] 0 (10 -3 M) The initial rate was determined for each of four reactions with different starting concentrations of A

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Initial Rates Method [A] 0 (10 -3 M) Rate (10 7 M s -1 ) When we start with more A , the initial rate is faster. The rate is clearly dependent on the [A]. 130 41 9.6 3.6 rate 0 (10 -7 M s -1 ) 30 17 8.2 5.0 [A] 0 (10 -3 M) Example : the initial rate of a reaction depended on the concentration of a substance A as follows:
Initial Rates Method Find the order of the reaction with respect to A and the rate constant ( k ) . what do we know? rate 0 = k [A] 0 n note that as [A] 0 goes up by 2, the rate 0 goes up by 2 2 130 41 9.6 3.6 rate 0 (10 -7 M s -1 ) 30 17 8.2 5.0 [A] 0 (10 -3 M) Example : the initial rate of a reaction depended on the concentration of a substance A as follows:

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Initial Rates Method what do we know? rate 0 = k [A] 0 n log rate 0 = log k + nlog [A] 0 plot this function 130 41 9.6 3.6 rate 0 (10 -7 M s -1 ) 30 17 8.2 5.0 [A] 0 (10 -3 M) Example : the initial rate of a reaction depended on the concentration of a substance A as follows: Find the order of the reaction with respect to A and the rate constant. note that as [A] 0 goes up by 2, the rate 0 goes up by 2 2
(log k ) Initial Rates Method log rate 0 = log k + nlog[A] 0 log rate 0 log [A] 0 2.000498 slope -0.84439 Intercept Coefficients (n) Slope = 2.0 = order Log k = -0.84; k=0.14 M -1 s -1 0.999973 R-squared Regression Statistics a word of caution: log-log plots tend to be linear, no matter what!

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17 - Rates - Chemical Kinetics Rate Laws a rate law is an...

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