1. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemical Kinetics The area of chemistry that concerns reaction rates.

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Main Goal of Chemical Kinetics To understand the steps by which a reaction takes place. That is, REACTION MECHANISM

3. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Reaction Rate Speed or rate: Change in concentration (conc) of a reactant or product per unit time.

4. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 [A] = conc. of A in moles/liter. Rate = + : increase in conc. Rate = - : decrease in conc.

5. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 2 NO 2 (g)  2NO (g) + O 2 (g) -- START WITH FLASK OF ONLY NO 2 [NO] = [O 2 ] = 0 -- AS TIME PASSES NO 2 is converted to NO and O 2

6. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Figure 12.2 Representation of the Reaction of 2NO 2 (g)  g) + O 2 (g)

7. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Rate is not constant, but decreases with time. 1 st 50 seconds: Rate = [NO 2 ] t =50 s – [NO 2 ] t=0 s 50 sec. – 0 sec. = - 4.2 x 10 -5 mole/ L sec

8. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Rate is not constant, but decreases with time. 2nd 50 seconds: Rate = ([NO 2 ] t =100 s – [NO 2 ] t=50 s) 100 sec. – 50 sec. = - 2.8 x 10 -5 mole/ L sec Recall 1 st 50 seconds Rate = - 4.2 x 10 -5 mole/ L sec

9. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 So far we have looked at rate of reaction in terms of the reactant. We can also look at it in terms of the PRODUCT. 2 NO 2 (g)  2NO (g) + O 2 (g) Rate of consumption of NO 2 = Rate of production of NO = 2 X Rate of production of O 2

10. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 -  [NO 2 ] = +  [NO] = + 2  [O 2 ]  t  t  t 2 NO 2 (g)  2NO (g) + O 2 (g) or

11. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 2 NO 2 (g)  2NO (g) + O 2 (g) -  [NO 2 ] = +  [NO] = +  [O 2 ] 2  t 2  t  t or

12. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 1)Rate depends on the reactant or product being studied. 2)Reaction rate changes with time because the concentrations change with time. 3)Be very specific when you describe a rate for a chemical reaction.

13. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 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.

14. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 A MODEL FOR CHEMICAL KINETICS COLLISION THEORY: ACTIVATION ENERGY ( E a ): MINIMUM ENERGY OF COLLISION REQUIRED FOR 2 MOLECULES TO REACT.

15. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 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.

16. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 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

17. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 (1) RATE OF RXN. DEPENDS ON TEMPERATURE. ROUGH RULE OF THUMB: IN MANY CASES, RATE DOUBLES (approx.) for every 10 °C Increase. Factors Affecting Reaction Rate

18. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 (2)Concentration: Increasing the conc. Causes increases collision frequency Factors Affecting Reaction Rate

19. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 (3)Particle size (Surface Area): Increases the amount of reactant exposed for the reaction thereby increasing the collision frequency. Factors Affecting Reaction Rate

20. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 ORIENTATION OF MOLECULES ALSO IMPORTANT DURING COLLISIONS. Observed Reaction Rates are still smaller than the rate of collisions with enough activation energy.

21. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Catalysis Catalyst: A substance that speeds up a reaction without being consumed Inhibitor: A substance that slows down a reaction or interfers with the action of a catalyst (“poisons the catalyst”)

22. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Catalysis Catalyst: A substance that speeds up a reaction without being consumed Enzyme: A large molecule (usually a protein) that catalyzes biological reactions.

23. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction How do They Work?

24. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Catalysts Lower Activation Energy, BUT does not affect the  E, energy difference between the products and the reactants.

25. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction

26. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Catalysts Homogeneous catalyst: Present in the same phase as the reacting molecules. Heterogeneous catalyst: Present in a different phase than the reacting molecules.

27. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 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

28. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Heterogeneous Catalysis of the Hydrogenation of Ethylene Main function of catalyst -- weaken the H—H bonds by formation of metal – H interactions.

29. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Catalytic Converter

30. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Homogeneous Catalysis Catalyst is in the same phase as the reacting molecule. Examples Freon

31. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 Enzymes: Nature’s Catalysts Enzymes are large molecules specifically tailored to facilitate a given type of reaction. Usually enzymes are proteins