1. MECHANISMS A Microscopic View of Reactions Sections 15.5 and 15.6 How are reactants converted to products at the molecular level? Want to connect the RATE LAW ----> MECHANISM experiment ---->theory

2. 15.5 A MICROSCOPIC VIEW OF REACTIONS The collision theory can be used to explain reaction rates.The collision theory can be used to explain reaction rates. In this theory, reactants must collide in order to react.In this theory, reactants must collide in order to react. One factor in determining if these collisions will be successful is the energy of the collision.One factor in determining if these collisions will be successful is the energy of the collision. The minimum energy required for reaction is called the activation energy.The minimum energy required for reaction is called the activation energy.

3. MICROSCOPIC VIEW OF REACTIONS Figure 15.15 illustrates the activation energy required for the conversion of cis to trans 2- butene.Figure 15.15 illustrates the activation energy required for the conversion of cis to trans 2- butene.Figure 15.15Figure 15.15 –This reaction is exothermic since the product is at a lower energy than the reactant. O.H. #63, #85, & Figure 15.14 illustrate this concept in a general way and show the relationship between the activation energy, E a, and the energy of the reaction, ΔE.O.H. #63, #85, & Figure 15.14 illustrate this concept in a general way and show the relationship between the activation energy, E a, and the energy of the reaction, ΔE.Figure 15.14Figure 15.14 The relative number of reactant molecules with energy equal to or greater than E a is illustrated in O.H. Figure 15.13, page 712.The relative number of reactant molecules with energy equal to or greater than E a is illustrated in O.H. Figure 15.13, page 712.

4. Figure 15.15

5. Figure 15.14

6. MECHANISMSMECHANISMS For example Rate = k [trans-2-butene] Conversion requires twisting around the C=C bond. H 3 C CC CH 3 H H H 3 C CC H H 3 trans-2-butene cis-2-butene

7. MECHANISMSMECHANISMS Conversion of trans to cis butene

8. Energy involved in conversion of trans to cis butene MECHANISMSMECHANISMS See Figure 15.15 trans cis energy Activated Complex 26.9 kJ/mol 30.2 kJ/mol 3.9 kJ/mol 233 kJ 229 kJ

9. Concentration The effect of concentration is seen in the rate equation. As might be expected, as the concentration increases, the rate increases.The effect of concentration is seen in the rate equation. As might be expected, as the concentration increases, the rate increases. If we assume the reaction between A and B occurs with the collision of A and B, then the reaction would be first order in A and first order in B.If we assume the reaction between A and B occurs with the collision of A and B, then the reaction would be first order in A and first order in B. The reaction rate would double if the concentration of A were doubled, and increased by a factor of four if both the concentration of A and B were doubled.The reaction rate would double if the concentration of A were doubled, and increased by a factor of four if both the concentration of A and B were doubled. Figure 15.12.Figure 15.12.

10. Figure 15.12 Concentration and collision frequency

11. Effects of Molecular Orientation Simple reactants have no particular orientation and are independent of this consideration.Simple reactants have no particular orientation and are independent of this consideration. More complex reactants may be very sensitive to the orientation of the molecules at the time of the collision.More complex reactants may be very sensitive to the orientation of the molecules at the time of the collision. Figure 15.16, page 715.Figure 15.16, page 715.

12. Figure 15.16

13. MECHANISMSMECHANISMS Reaction passes thru a TRANSITION STATE where there is an activated complex that has sufficient energy to become a product. ACTIVATION ENERGY, E a = energy required to form activated complex. Here E a = 233 kJ/mol trans cis energy Activated Complex 26.9 kJ/mol 30.2 kJ/mol 3.9 kJ/mol 233 kJ 229 kJ

14. Also note that trans-butene is MORE STABLE than cis-butene by about 4 kJ/mol. Also note that trans-butene is MORE STABLE than cis-butene by about 4 kJ/mol. Therefore, trans ---> cis is ENDOTHERMIC. Therefore, trans ---> cis is ENDOTHERMIC. This is the connection between thermo- dynamics and kinetics. This is the connection between thermo- dynamics and kinetics. MECHANISMSMECHANISMS trans cis energy Activated Complex 26.9 kJ/mol 30.2 kJ/mol 3.9 kJ/mol 233 kJ 229 kJ

15. Activation Energy A flask full of trans-butene is stable because only a tiny fraction of trans molecules have enough energy to convert to cis. A flask full of trans-butene is stable because only a tiny fraction of trans molecules have enough energy to convert to cis. In general, differences in activation energy are the reason reactions vary from fast to slow. In general, differences in activation energy are the reason reactions vary from fast to slow.

16. MECHANISMSMECHANISMS 1. Why is reaction observed to be 1st order? As [trans] doubles, number of molecules with enough E also doubles. As [trans] doubles, number of molecules with enough E also doubles. 2. Why is the reaction faster at higher temperature? Fraction of molecules with sufficient activation energy increases with T. Fraction of molecules with sufficient activation energy increases with T.

17. MECHANISMS Reaction of trans --> cis is UNIMOLECULAR - only one reactant is involved. BIMOLECULAR — two different molecules must collide --> products A bimolecular reaction Exo- or endothermic?

18. The Arrhenius Equation Many reactions have a very significant temperature dependence, even doubling or tripling with a ten degree change in temperature.Many reactions have a very significant temperature dependence, even doubling or tripling with a ten degree change in temperature. The primary temperature dependence is given by the Arrhenius equationThe primary temperature dependence is given by the Arrhenius equation RT E a Aek   RT E a e  “A” is the pre-exponential factor and is related to the collision frequency corrected for orientation considerations. The fraction of collisions with energy E > E a is

19. Arrhenius Equation A plot of ln k vs. 1/T has a slope, m, withA plot of ln k vs. 1/T has a slope, m, with – m = -E a /R, and a y-intercept of ln A. –(Our text uses b for slope and a for y- intercept.) –We will calculate the activation energy in the lab using this method. The two-point form of the equation is:The two-point form of the equation is:

20. 15.6 Reaction Mechanisms The actual sequence of steps by which the reaction proceeds in a multi-step process is called the reaction mechanism.The actual sequence of steps by which the reaction proceeds in a multi-step process is called the reaction mechanism. This sequence of steps is seldom known so most mechanisms are simply guesses that fit all we know about the reaction.This sequence of steps is seldom known so most mechanisms are simply guesses that fit all we know about the reaction. The individual steps are called elementary steps.The individual steps are called elementary steps. The number of molecules involved in a step is called the molecularity of that step and can be determined by counting based on the proposed mechanism.The number of molecules involved in a step is called the molecularity of that step and can be determined by counting based on the proposed mechanism.

21. Reaction Mechanisms A ====> product, is unimolecular. A + B ===> product, is bimolecular. A + 2 B =====> product, is termolecular. Unimolecular and bimolecular processes are common, but termolecular processes are very rare.Unimolecular and bimolecular processes are common, but termolecular processes are very rare. The rate equation for an elementary step can be written using the molecularity of that step.The rate equation for an elementary step can be written using the molecularity of that step. We see that for elementary steps the molecularity and the order are the same.We see that for elementary steps the molecularity and the order are the same.

22. Reaction Mechanisms The value of the constant, k, however, cannot be determined and is left in symbol form.The value of the constant, k, however, cannot be determined and is left in symbol form. A mechanism is considered feasible if the sum of the elementary steps is stoichiometrically consistent and the order of the reaction is consistent with the order derived from the elementary steps.A mechanism is considered feasible if the sum of the elementary steps is stoichiometrically consistent and the order of the reaction is consistent with the order derived from the elementary steps. The slow step or rate-determining step is use to determine the order of the reaction.The slow step or rate-determining step is use to determine the order of the reaction.

23. Collision Theory Reactions require (a) activation energy and (b) correct geometry. O 3 (g) + NO(g) O 2 (g) + NO 2 (g) O 3 (g) + NO(g) O 2 (g) + NO 2 (g) 2. Activation energy and geometry 1. Activation energy

24. MECHANISMSMECHANISMS O 3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a sequence of elementary steps. O 3 + NO reaction occurs in a single ELEMENTARY step. Most others involve a sequence of elementary steps. Adding elementary steps gives NET reaction. Adding elementary steps gives NET reaction.

25. MECHANISMSMECHANISMS Most reactions involve a sequence of elementary steps. 2 I - + H 2 O 2 + 2 H + ---> I 2 + 2 H 2 O 2 I - + H 2 O 2 + 2 H + ---> I 2 + 2 H 2 O Rate = k [I - ] [H 2 O 2 ] Rate = k [I - ] [H 2 O 2 ] Step 1 — slowHOOH + I - --> HOI + OH - Step 2 — fastHOI + I - --> I 2 + OH - Step 3 — fast2 OH - + 2 H + --> 2 H 2 O Rate of the reaction controlled by slow step — RATE DETERMINING STEP, rds. RATE DETERMINING STEP, rds. Rate can be no faster than rds!

26. MECHANISMSMECHANISMS Step 1 is bimolecular and involves I - and HOOH. Therefore, this predicts the rate law should be Rate = k[I - ] [H 2 O 2 ] — as observed!! The species HOI and OH - are reaction intermediates. 2 I - + H 2 O 2 + 2 H + ---> I 2 + 2 H 2 O Rate = k [I - ] [H 2 O 2 ] Rate = k [I - ] [H 2 O 2 ] Step 1 — slowHOOH + I - --> HOI + OH - Step 2 — fastHOI + I - --> I 2 + OH - Step 3 — fast2 OH - + 2 H + --> 2 H 2 O

27. Reaction Mechanisms 2 NO + 2 H 2 ===> N 2 + 2 H 2 O, d(N 2 )/dt = k [NO] 2 [H 2 ] Can this reaction occur in one step?Can this reaction occur in one step? Is the following mechanism feasible? If so, how?Is the following mechanism feasible? If so, how?

28. Reaction Mechanisms K 2 NO N 2 O 2, fast equilibrium step k 1 k 1 N 2 O 2 + H 2 ====> 2 NOH, slow step k 2 k 2 2 NOH ====> N 2 + 2 OH, fast step k 3 k 3 OH + H 2 ====> H 2 O + H, fast step k 4 k 4 H + OH ====> H 2 O, fast step

29. CATALYSTS AND REACTION RATE Catalysts are substances that alter the reaction rate without being consumed.Catalysts are substances that alter the reaction rate without being consumed. They function by altering the reaction mechanism.They function by altering the reaction mechanism. The new mechanism has a different activation energy.The new mechanism has a different activation energy. Catalysts are of two varieties, homogeneous and heterogeneous.Catalysts are of two varieties, homogeneous and heterogeneous. Catalysts play a very significant role in most commercial chemical processes.Catalysts play a very significant role in most commercial chemical processes.

30. Figure 15.18

31. Iodine-Catalyzed Isomerization of cis- 2-Butene Figure 15.19

32. CATALYSISCATALYSIS Catalysts speed up reactions by altering the mechanism to lower the activation energy barrier. What is a catalyst? Catalysts and society Dr. James Cusumano, Catalytica Inc.

33. CATALYSIS In auto exhaust systems — Pt, NiO 2 CO + O 2 ---> 2 CO 2 2 NO ---> N 2 + O 2

34. CATALYSIS 2.Polymers: H 2 C=CH 2 ---> polyethylene 3.Acetic acid: CH 3 OH + CO --> CH 3 CO 2 H 4. Enzymes — biological catalysts

35. CATALYSIS Catalysis and activation energy Uncatalyzed reaction Catalyzed reaction MnO 2 catalyzes decomposition of H 2 O 2 2 H 2 O 2 ---> 2 H 2 O + O 2

36. MnO 2 Catalyzed Decomposition of H 2 O 2