CH14C
14-8 Theoretical Models for Chemical Kinetics Collision Theory Kinetic-Molecular theory can be used to calculate the collision frequency. In gases 1030 collisions per second. If each collision produced a reaction, the rate would be about 106 M s-1. Actual rates are on the order of 104 M s-1. ◦ Still a very rapid rate. Only a fraction of collisions yield a reaction. Slide 1 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Activation Energy For a reaction to occur there must be a redistribution of energy sufficient to break certain bonds in the reacting molecule(s). Activation Energy: The minimum energy above the average kinetic energy that molecules must bring to their collisions for a chemical reaction to occur. Slide 2 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Activation Energy Slide 3 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Kinetic Energy Slide 4 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Collision Theory If activation barrier is high, only a few molecules have sufficient kinetic energy and the reaction is slower. As temperature increases, reaction rate increases. Orientation of molecules may be important. Slide 5 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Collision Theory Slide 6 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Transition State Theory The activated complex is a hypothetical species lying between reactants and products at a point on the reaction profile called the transition state. Slide 7 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 14-9 Effect of Temperature on Reaction Rates Svante Arrhenius demonstrated that many rate constants vary with temperature according to the equation: k = Ae-Ea/RT ln k = -Ea 1 R Slide 8 of 61 + lnA T General Chemistry: Chapter 14 Prentice-Hall © 2007 Arrhenius Plot N2O5(CCl4) → N2O4(CCl4) + ½ O2(g) -Ea = -1.2104 K R -Ea = 1.0102 kJ mol-1 Slide 9 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Arrhenius Equation k= R ln R + ln A T -Ea 1 ln k2– ln k1 = k1 k2 Slide 10 of 61 -Ea 1 ln k = Ae-Ea/RT = 1 -E a + ln A - ln A T2 R T1 -Ea 1 R T2 - 1 T1 General Chemistry: Chapter 14 Prentice-Hall © 2007 14-10 Reaction Mechanisms A step-by-step description of a chemical reaction. Each step is called an elementary process. Any molecular event that significantly alters a molecules energy of geometry or produces a new molecule. Reaction mechanism must be consistent with: Stoichiometry for the overall reaction. The experimentally determined rate law. Slide 11 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Elementary Processes Unimolecular or bimolecular. Exponents for concentration terms are the same as the stoichiometric factors for the elementary process. Elementary processes are reversible. Intermediates are produced in one elementary process and consumed in another. One elementary step is usually slower than all the others and is known as the rate determining step. Slide 12 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Slow Step Followed by a Fast Step H2(g) + 2 ICl(g) → I2(g) + 2 HCl(g) d[P] dt = k[H2][ICl] Postulate a mechanism: H2(g) + ICl(g) slow d[HI] HI(g) + HCl(g) HI(g) + ICl(g) fast I2(g) + HCl(g) H2(g) + 2 ICl(g) → I2(g) + 2 HCl(g) Slide 13 of 61 General Chemistry: Chapter 14 dt d[I2] dt d[P] dt = k[H2][ICl] = k[HI][ICl] = k[H2][ICl] Prentice-Hall © 2007 Slow Step Followed by a Fast Step Slide 14 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Fast Reversible Step Followed by a Slow Step 2NO(g) + O2(g) → 2 NO2(g) d[P] dt = -kobs[NO2]2[O2] Postulate a mechanism: k1 fast 2NO(g) K= slow k-1 k1 k-1 = N2O2(g) [N2O2] = k-1 [NO]2 = K[NO]2 [N2O2] [NO] N2O2(g) + O2(g) k2 2NO2(g) 2NO(g) + O2(g) → 2 NO2(g) Slide 15 of 61 k1 General Chemistry: Chapter 14 d[NO2] dt d[I2] dt = k2[N2O2][O2] = k2 k1 k-1 [NO]2[O2] Prentice-Hall © 2007 Catalytic Converters Dual catalyst system for oxidation of CO and reduction of NO. cat CO + Slide 16 of 61 NO CO2 + N2 General Chemistry: Chapter 14 Prentice-Hall © 2007 14-5 Catalysis Slide 17 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Worked Examples Follow: Slide 18 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Slide 19 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Slide 20 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Slide 21 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 CRS Questions Follow: Slide 22 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 1. Energy Energy A+B reaction coordinate reaction coordinate 4. Energy 3. A+B X+Y reaction coordinate Slide 23 of 61 X+Y X+Y Energy The reaction between A and B is determined to be a fairly fast reaction and slightly exothermic. Which of the following potential energy surfaces fit this description? 2. A+B General Chemistry: Chapter 14 A+B X+Y reaction coordinate Prentice-Hall © 2007 1. Energy Energy A+B reaction coordinate reaction coordinate 4. Energy 3. A+B X+Y reaction coordinate Slide 24 of 61 X+Y X+Y Energy The reaction between A and B is determined to be a fairly fast reaction and only slightly exothermic. Which of the following potential energy surfaces fit this description? 2. A+B General Chemistry: Chapter 14 A+B X+Y reaction coordinate Prentice-Hall © 2007 Energy A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol-1, respectively. The enthalpy change for the reaction is, (do not use a calculator) R 1. DH = 60 kJ mol-1 2. DH = -60 kJ mol-1 P 3. DH = 80 kJ mol-1 4. DH = -80 kJ mol-1 reaction coordinate 5. DH = 140 kJ mol-1 Slide 25 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Energy A particular reaction was found to have forward and reverse activation energies of 60 and 140 kJ mol-1, respectively. The enthalpy change for the reaction is, (do not use a calculator) R 1. DH = 60 kJ mol-1 2. DH = -60 kJ mol-1 P 3. DH = 80 kJ mol-1 4. DH = -80 kJ mol-1 reaction coordinate 5. DH = 140 kJ mol-1 Slide 26 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007 Energy A+B X+Y reaction coordinate X+Y 4. A+B X+Y reaction coordinate Slide 27 of 61 A+B reaction coordinate General Chemistry: Chapter 14 Energy 3. Energy In which diagram to the right does the dashed line best represent the catalyzed version of the reaction’s potential energy profile? Energy 2. 1. X+Y A+B reaction coordinate Prentice-Hall © 2007 Energy A+B X+Y reaction coordinate X+Y 4. A+B X+Y reaction coordinate Slide 28 of 61 A+B reaction coordinate General Chemistry: Chapter 14 Energy 3. Energy In which diagram to the right does the dashed line best represent the catalyzed version of the reaction’s potential energy profile? Energy 2. 1. X+Y A+B reaction coordinate Prentice-Hall © 2007 Textbook End of Chapter ?’s: P.611- #1, 3, 11, 13, 17, 19, 21, 33, 47, 51, 55, 100, 101, 102, 103, 104, 105 Slide 29 of 61 General Chemistry: Chapter 14 Prentice-Hall © 2007
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