1. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Chemistry FIFTH EDITION Chapter 12 Chemical Kinetics Schedule: http://www2.fultonschools.org/teacher/warrene/ http://www2.fultonschools.org/teacher/warrene/

3. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Chemical Kinetics The area of chemistry that concerns reaction rates.

4. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Main Goal of Chemical Kinetics To understand the steps by which a reaction takes place. That is, REACTION MECHANISM

5. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Section 12.6 REACTION MECHANISMS MOST CHEMICAL RXNS. OCCUR BY A SERIES OF STEPS called the REACTION MECHANISM EACH STEP IN THE SERIES IS A SINGLE MOLECULAR EVENT called an ELEMENTARY REACTION. THE SET OF ELEMENTARY REACTIONS IS THE REACTION MECHANISM.

6. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Overall Balanced Equation: NO 2 (g) + CO (g)  NO (g) + CO 2 (g) Reaction Mechanism: Step 1: NO 2 + NO 2 (g)  NO 3 (g) + NO (g) Step 2: NO 3 + CO (g)  NO 2 (g) + CO 2 (g)

7. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Figure 12.9 A Molecular Representation of the Elementary Steps in the Reaction of NO 2 and CO

8. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 The Sum of the Set of Elementary Reactions GIVES the Overall Effect represented by a NET Chemical Equation. What is happening on the Molecular Level is best represented by the Reaction Mechanism and is often more Complicated than that represented by the Simple Chemical Equation.

9. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 REACTION INTERMEDIATE SPECIES PRODUCED DURING A RXN. THAT DOES NOT APPEAR IN THE NET EQUATION BECAUSE IT REACTS IN A SUBSEQUENT STEP IN THE MECHANISM.

10. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure 12.9 A Molecular Representation of the Elementary Steps in the Reaction of NO 2 and CO NO 3 NO 3 is a reaction intermediate.

11. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 ELEMENTARY STEPS RATE LAW DEPENDS ON MOLECULARITY (i.e., the # of species that must collide to produce the reaction indicated by that step.) UNIMOLECULAR STEP: Rxn. involving one molecule; always 1 st order. BIMOLECULAR: Rxn by the collision of 2 species; Always 2 nd order.

12. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 TERMOLECULAR: Rxn. by the collision of 3 species; Very rare Probability of 3 molecules colliding simultaneously is very small. See Table 12.7 on page 550. Examples of Elementary Steps.

13. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 REACTION MECHANISM SERIES OF ELEMENTARY STEPS THAT MUST SATISFY TWO REQUIREMENTS: 1.SUM OF ELEMENTARY STEPS MUST GIVE THE OVERALL BALANCED EQUATION FOR THE REACTION. 2.MECHANISM MUST AGREE WITH THE EXPERIMENTALLY DETERMINED RATE LAW.

14. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Overall Balanced Equation: NO 2 (g) + CO (g)  NO (g) + CO 2 (g) Reaction Mechanism: Step 1: NO 2 + NO 2 (g)  NO 3 (g) + NO (g) Step 2: NO 3 + CO (g)  NO 2 (g) + CO 2 (g) First Requirement is met.

15. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 RATE OF THE REACTION In Multi-step reactions, reaction is only as fast as its slowest step. RATE IS ONLY DEPENDENT ON THE SLOWEST STEP, KNOWN AS THE RATE DETERMINING STEP.

16. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 In Proposed Mechanism, assume Step 1 is the slow step, the rate determining step. assume Step 2 is a fast step. Reaction Mechanism: Step 1: NO 2 + NO 2 (g)  NO 3 (g) + NO (g) Step 2: NO 3 + CO (g)  NO 2 (g) + CO 2 (g)

17. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 For Step 1 (an elementary step), Rate of formation of NO 3 =  [NO 3 ] = k [NO 2 ] 2  t This agrees with experimentally determined Rate law. Rate Mechanism may be correct (not proven.)

18. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Let’s do #49, 51 & 52 on page 5571 Read Handout The Rate Law and the Mechanism Do Exercise 14.12 Put in folder on Friday January 16.

19. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 RATE OF RXN. DEPENDS ON TEMPERATURE. ROUGH RULE OF THUMB: IN MANY CASES, RATE DOUBLES (approx.) for every 10 °C Increase. Section 12.7 A Model for Chemical Kinetics

20. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Figure 12.10 A Plot Showing the Exponential Dependence of the Rate Constant on Absolute Temperature

21. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 A MODEL FOR CHEMICAL KINETICS COLLISION THEORY: IN ORDER FOR A RXN. TO OCCUR, REACTANT MOLECULES MUST COLLIDE WITH (1)AN ENERGY GREATER THAN SOME MINIMUM VALUE (2) AND WITH PROPER ORIENTATION. ACTIVATION ENERGY ( E a ): MINIMUM ENERGY OF COLLISION REQUIRED FOR 2 MOLECULES TO REACT.

22. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Collision Theory Key Idea: Molecules must collide to react. However, only a small fraction of collisions produces a reaction. Why? Arrhenius: An activation energy must be overcome.

23. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 CONSIDER REACTION 2BrNO (g)  2 NO (g) + Br 2 (g) Energy comes from the KE possessed by the reacting molecules before they collide. During the collision, KE changed to PE & used to distort molecules, break bonds & rearrange atoms.

24. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Figure 12.11 Change in Potential Energy Top of PE Hill: Activated complex or Transition State which is the arrangement of atoms found at the top of the PE Hill Exothermic

25. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Figure 12.12 Plot Showing the Number of Collisions with a Particular Energy at T 1 and T 2, where T 2  

26. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 ORIENTATION OF MOLECULES ALSO IMPORTANT DURING COLLISIONS. Observed Reaction Rates are still smaller than the rate of collisions with enough activation energy.

27. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Figure 12.13 Several Possible Orientations for a Collision Between Two BrNO Molecules Some collision orientations lead to rxn & other do not!

28. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 REQUIREMENTS FOR REACTANTS TO COLLIDE & SUCCESSFULLY REARRANGE TO FORM PRODUCTS 1) COLLISION ENERGY MUST EQUAL OR EXCEED THE ACTIVATION ENERGY. 2) RELATIVE ORIENTATION OF REACTANTS MUST ALLOW FORMATION OF ANY NEW BONDS NECESSARY TO PRODUCE THE PRODUCTS.

29. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 ARRHENIUS EQUATION k = A e -Ea/RT A = z p = frequency factor where z = collision frequency (changes slowly with temp). p = steric factor, reflects the fraction of collisions with effective orientations. k = rate constant Ea = activation energy T = temperature R = gas constant

30. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 e -Ea/RT Fraction of collisions with sufficient energy to produce a reaction. Changes rapidly with Temperature.

31. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 ARRHENIUS EQUATION ln (k) = [-(E a /R) (1/T) ] + ln A Plot of ln k versus 1/T gives a straight line. Slope = -E a /R Y- intercept = ln A

32. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 Figure 12.14 Plot of ln(k) Versus 1/T for the Reaction 2N 2 O 5 (g)     g) + O 2 (g) Slope = - E a /R

33. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 ARRHENIUS EQUATION (Another Form) ln (k 2 /k 1 ) = E a /R [ 1/T 1 – 1/T 2 ] E a can be calculate from values of k at two different temperatures. Homework: #53-57 all, 59-63 all

34. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Section 12.8 Catalysis Catalyst: A substance that speeds up a reaction without being consumed Read Write a detailed summary –

35. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35 Section 12.8 Catalysis Catalyst: A substance that speeds up a reaction without being consumed Enzyme: A large molecule (usually a protein) that catalyzes biological reactions.

36. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 36 Figure 12.15 Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction How do They Work?

37. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 37 Figure 12.16 Effect of a Catalyst on the Number of Reaction-Producing Collisions

38. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 38 Catalysts Lower Activation Energy, BUT does not affect the  E, energy difference between the products and the reactants.

39. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 39 Figure 12.15 Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction

40. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 40 Catalysts Homogeneous catalyst: Present in the same phase as the reacting molecules. Heterogeneous catalyst: Present in a different phase than the reacting molecules.

41. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 41 Heterogeneous Catalysts -- most often involves gaseous reactants being adsorbed on the surface of a solid catalyst. EXAMPLE: Hydrogenation of ethylene H 2 C==CH 2 + H 2  H 3 C—CH 3

42. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 42 Figure 12.17 Heterogeneous Catalysis of the Hydrogenation of Ethylene Main function of catalyst -- weaken the H—H bonds by formation of metal – H interactions.

43. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 43 Heterogeneous Catalysis 1. Adsorption and activation of the reactants. 2. Migration of the adsorbed reactants on the surface. 3. Reaction of the adsorbed substances. 4. Escape, or desorption, of the products. Steps:

44. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 44 Other Examples of Heterogeneous Catalysis (1) Oxidation of SO 2 (g) and SO 3 (g) (2)Catalytic Converter for Automobile Exhaust Solid catalyst is a mixture of catalysts

45. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 45 Figure 12.18 Catalytic Converter

46. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 46 Homogeneous Catalysis Catalyst is in the same phase as the reacting molecule. Examples (1) NO (2) Freon

47. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 47 Enzymes: Nature’s Catalysts Enzymes are large molecules specifically tailored to facilitate a given type of reaction. Usually enzymes are proteins, biomolecules constructed from  -amino acids See page 596 Proteins: “Polymers of amino acids” Body makes specific proteins from amino acids that come from the proteins that we eat.

48. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 48 Figure 12.19 The Removal of the End Amino Acid from a Protein by Reaction with a Molecule of Water

49. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 49 Figure 12.20 The Structure of the Enzyme Carboxypeptidase-A

50. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 50 Enzyme, Carboxypeptidase-A catalyzes this reaction. Homework: Do it!!