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Kinetics …or Reaction Rates

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Change The ice melted. The nail rusted.

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Expressing Change The ice melted. The nail rusted. mL/min g/week

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Expressing Change Reaction Rate. Change in something divided by change in time amount time

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Reaction Rate As a rxn occurs, what happens to the amount of reactant? Rate = – reactant amt time

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Reaction Rate Consider: Mg(s) + 2HCl(aq) MgCl 2 (aq) + H 2 (g) Rate = – mol Mg = – mol HCl = t2 t mol MgCl 2 = mol H 2 t t

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Reaction Rate of a one-way reaction Will be determined by looking at the reactants only Decreases as time progresses. Why? What eventually happens to the amount of reactant?

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Reaction Rate of an equilibrium rxn Will be determined by looking at the reactants only Decreases as time progresses. Why? What eventually happens to the rate? Does the reaction stop?

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Reaction Rate 2NO 2 (g) 2NO(g) + O 2 (g)

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Reaction Rate Time (s) [Concentration] (mol/L) NO 2 NO O2O2

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Necessities of Reaction Proper orientation 2HI H 2 + I 2

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Necessities of Reaction Sufficient energy--Activation energy (E a ) Rxn progress Energy EaEa 2HI H 2 + I 2 Activated Complex

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Factors that affect Reaction Rate Concentration Temperature Surface Area Presence of a catalyst Rate-determining step…or the slowest step in a rxn mechanism

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Rate Law Relates the rate as a function of the reactant(s) Units of rate are always mol/L-s unless otherwise stated Two types: –Differential –Integrated

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Differential Rate Law For the rxn: A B…differential Rate = k[A] x k is the rate law constant…units are dependent on the order x is the order of reactant A

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Order may not be determined by looking at the coefficients of the reactants unless the rxn is the rate- determining step must be determined experimentally

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Order If the rate does not change when the reactant is doubled, then the order is zero; and rate is dependent solely on the value of k Rate = k[A] 0 or Rate = k

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Order If the rate doubles when the reactant is doubled, then the order is first; and rate is dependent on the concentration of the reactant Rate = k[A] 1

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Order If the rate quadruples when the reactant is doubled, then the order is second; and rate is dependent on the square of the concentration of the reactant Rate = k[A] 2

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Overall Order of Reaction is the sum of the individual orders When Rate = k[A] 0, the overall order is zero When Rate = k[A] 1, the overall order is one

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Overall Order of Reaction If a differential rate law for the following rxn: A + B C is Rate = k[A] 1 [B] 2 then the overall order is three –What must the units of k be in this reaction?

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Differential Rate Law For the rxn: I 1- + OCl 1- IO 1- + Cl 1- The following data were collected: T[I 1- ] o [OCl 1- ] o Rate(mol/L-s) 10.1200.1800.0791 20.0600.1800.0395 30.0300.0900.00988 40.2400.0900.0791 50.030 0.00329

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Differential Rate Law a.Determine the differential rate law. b.Determine the overall order of reaction. c.Determine the value of the rate law constant with its units.

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Differential Rate Law a.Trial #1Rate = k[I 1- ] x [OCl 1- ] y Trial #2Rate = k[I 1- ] x [OCl 1- ] y 0.0791 = k[0.12] x [0.18] y 0.0395 = k[0.06] x [0.18] y

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Differential Rate Law 2.00 = [2] x log 2.00 = x(log 2) log 2.00 = x log 2 x =1

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Differential Rate Law a.Trial #3Rate = k[I 1- ] x [OCl 1- ] y Trial #5Rate = k[I 1- ] x [OCl 1- ] y 0.00988 = k[0.03] x [0.09] y 0.00329 = k[0.03] x [0.03] y

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Differential Rate Law 3.00 = [3] y log 3.00 = y(log 3) log 3.00 = y log 3 y =1

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Differential Rate Law a. Rate = k[I 1- ] 1 [OCl 1- ] 1

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Differential Rate Law b. 1 + 1 = 2 overall order

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Differential Rate Law c.0.00988mol = k[0.03mol] 1 [0.09mol] 1 L-s L L 0.00988mol = k 0.0027mol 2 L-s L 2 3.66 L/mol-s

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Differential Rate Law For the rxn: A + B + C products The following data were collected: T [A] o [B] o [C] o Rate(mol/L-s) 10.4000.3000.5607.14 x 10 -4 20.1000.5000.2004.55 x 10 -5 30.1000.200 4.55 x 10 -5 40.4000.3000.7501.28 x 10 -3 50.1000.3000.5603.57 x 10 -4

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Differential Rate Law a.Determine the differential rate law. b.Determine the overall order of reaction. c.Determine the value of the rate law constant with its units.

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Differential Rate Law a.Rate = k[A] 0.5 [B] 0 [C] 2 b. 0.5 + 0 + 2 = 2.5 c. 0.0036L 1.5 /mol 1.5 s

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Reaction Mechanism Most reactions do not occur in a single step Rather, they happen in a series of steps called elementary steps The sum of the elementary steps gives the overall reaction.

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Reaction Mechanism Intermediates are substances that are formed in one elementary step and consumed in a subsequent elementary step. They are rarely part of the rate law. Catalysts are substances added to a step that are also produced in a subsequent step. They are rarely part of the rate law.

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Reaction Mechanism Rate-determining step is the slowest step in a mechanism. –The differential rate law may be written from the reactants in the rate-determining step.

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Reaction Mechanism Consider the following elementary steps: Step 1: IBr(g) I(g)+Br(g) fast Step 2: IBr(g)+Br(g) I(g )+Br 2 (g) slow Step 3: I(g)+I(g) I 2 (g) fast a. What is the overall balanced equation? b. What is the differential rate law? c. What substance(s) is an intermediate? a catalyst?

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Reaction Mechanism a.2IBr(g) Br 2 (g) + I 2 (g) b. rate = k[IBr] 2, since [Br] is not a part of the overall balanced equation, substitute [IBr] for [Br] c. Intermediates are I(g) and Br2(g). There is no catalyst.

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Chemical Kinetics © 2009, Prentice-Hall, Inc. Reaction Mechanisms Reactions may occur all at once or through several discrete steps. Each of these processes.

Chemical Kinetics © 2009, Prentice-Hall, Inc. Reaction Mechanisms Reactions may occur all at once or through several discrete steps. Each of these processes.

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