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Kinetics …or Reaction Rates. Change The ice melted. The nail rusted.

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Presentation on theme: "Kinetics …or Reaction Rates. Change The ice melted. The nail rusted."— Presentation transcript:

1 Kinetics …or Reaction Rates

2 Change The ice melted. The nail rusted.

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

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

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

6 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

7 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?

8 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?

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

10 Reaction Rate Time (s) [Concentration] (mol/L)  NO 2  NO O2O2

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

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

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

14 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

15 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

16 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

17 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

18 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

19 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

20 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

21 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?

22 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)

23 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.

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

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

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

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

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

29 Differential Rate Law b = 2  overall order

30 Differential Rate Law c mol = k[0.03mol] 1 [0.09mol] 1 L-s L L mol = k mol 2 L-s L L/mol-s

31 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) x x x x x 10 -4

32 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.

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

34 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.

35 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.

36 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.

37 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?

38 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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