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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Chemistry FIFTH EDITION Chapter 12 Chemical Kinetics Schedule:

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Let’s do #45 & 47 on page 605 Read Handout The Rate Law and the Mechanism Do Exercise 14.12

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Figure A Plot Showing the Exponential Dependence of the Rate Constant on Absolute Temperature

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 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.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 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.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 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.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Figure 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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure Plot Showing the Number of Collisions with a Particular Energy at T 1 and T 2, where T 2  

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 ORIENTATION OF MOLECULES ALSO IMPORTANT DURING COLLISIONS. Observed Reaction Rates are still smaller than the rate of collisions with enough activation energy.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Figure Several Possible Orientations for a Collision Between Two BrNO Molecules Some collision orientations lead to rxn & other do not!

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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 e -Ea/RT Fraction of collisions with sufficient energy to produce a reaction. Changes rapidly with Temperature.

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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Figure Plot of ln(k) Versus 1/T for the Reaction 2N 2 O 5 (g)     g) + O 2 (g) Slope = - E a /R

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 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. Let’s look at # 49, 50, 55, & 57 Homework: WebAssign 12.6 – 12.8

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Section 12.8 Catalysis Catalyst: A substance that speeds up a reaction without being consumed Read pages 592 – 599 Write a detailed summary –

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 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.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Figure Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction How do They Work?

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Figure Effect of a Catalyst on the Number of Reaction-Producing Collisions

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Catalysts Lower Activation Energy, BUT does not affect the  E, energy difference between the products and the reactants.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Figure Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Catalysts Homogeneous catalyst: Present in the same phase as the reacting molecules. Heterogeneous catalyst: Present in a different phase than the reacting molecules.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Figure Heterogeneous Catalysis of the Hydrogenation of Ethylene Main function of catalyst -- weaken the H—H bonds by formation of metal – H interactions.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 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:

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

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Figure Catalytic Converter

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 Homogeneous Catalysis Catalyst is in the same phase as the reacting molecule. Examples (1) NO (2) Freon

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 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.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Figure The Removal of the End Amino Acid from a Protein by Reaction with a Molecule of Water

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Enzyme, Carboxypeptidase-A catalyzes this reaction. Homework: # 59 – 63, 64a, b.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35 Figure The Structure of the Enzyme Carboxypeptidase-A