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Chapter 1

Chemical Kinetics and Reaction Mechanisms. Chapter 1. Reactions and Reaction Rates. Prof. Kyoung -Ho Park. Prepared from Chemical Kinetics and Mechanism, 2 nd Ed. James H. Espenson. 1.1 Reactin Mechnisms. Chemical kinetics; Study of the rates of chemical reaction:

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Chapter 1

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  1. Chemical Kinetics and Reaction Mechanisms Chapter 1 Reactions and Reaction Rates Prof. Kyoung-Ho Park Prepared from Chemical Kinetics and Mechanism, 2nd Ed. James H. Espenson

  2. 1.1 ReactinMechnisms • Chemical kinetics; Study of the rates of chemical reaction: - concentration as a function of time - pH, temperature, pressure, isotopic substitution, salt concentration, and so on.

  3. 1.2 Net Reactions and Reaction Rates Reaction equation; 2Fe2+ + Tl3+ = 2Fe3+ + Tl+ Thermodynamics - net equation (K) K1 = [Fe3+]2[Tl3]/[Fe2+]2[Tl3+] Kinetics - Reaction rates (v) V1 = k[Fe2+][Tl3+] not [Fe2+]2[Tl3+] ※The rate of a chemical reaction is always taken as a positive quantity, and the rate constant k is always positive as well.

  4. 1.3 Elementary Reactions and Reaction Mechanisms • Reaction mechanism: The collection of the individual steps by which the net reaction proceeds. • Elementary reactions: The constituent step of a composite mechanism ; the simplest possible molecular combinations. Generally unimolecular or bimolecular, Occasionally termolecular step Termolecularreaction: an elementary reaction involving the simultaneous collision of any combination of three molecules, atoms, or ions.

  5. Consider the combination of two methyl radicals, Eq. (1-7). 2CH3 → C2H6 (1-7) - ½d[CH3]/dt = k1[CH3]2 d[C2H6]/dt = k2[CH3]2 k1 = 2k2

  6. Elementary Reaction[eq. 1-9]

  7. 1.4 The Order of a Rate Law • Order of a reaction: The sum of the exponents of the concentration factors in the rate law 5Br- + BrO3- + 6H+ = 3Br2+ 3H2O (1-11) V = - d[BrO3-]/ dt = k[BrO3-][Br-][H+]2 Overall, the reaction is fourth-order

  8. J. Am. Chem. Soc., 1952, 74 (8), 2027-2031 Kinetics of the Polymerization of Methyl Methacrylate with Aliphatic Azobisnitriles as Initiators BY L. M. ARNETT • Abstract: Methyl methacrylate has been polymerized in bulk and in benzene solution with each of three aliphatic azobisnitriles which have decomposition rates in a range whose extremes differ by a factor of 100. The initial rate of polymerization is found to be proportional to the square root of the initiator concentration, to the square root of the specific rate constant for initiator decomposition, and to the first power of the monomer concentration. The relationship between the number average molecular weight and the initiator concentration indicates that approximately one-half of the radicals generated by decomposition of the initiator start polymer chains. It is also indicated that the termination reaction is combination of two growing chains.

  9. The Polymerization of methyl methacrylate

  10. Ligand Substitution Reaction

  11. 1.5 Factors Influencing Reaction Rates • Concentration : One of the reactants will accelerate the reaction. • Physical condition : Temperature and pressure affect rates. • The intensity of absorbed radiation : Sunlight or room lights may alter the rate of a reaction. • Properties of the solvent : Reaction rates will differ with the solvent’s polarity, viscosity, donor number etc.

  12. Transition state or activated complexes, - The entropy of activation : ΔS≠ - The enthalpy of activation : ΔH≠ - The Gibbs free of activation : ΔG≠ - The energy of activation : Ea - The volume of activation : ΔV≠ • Rate-controlling step (RSC); a single key step (slow reaction) in the mechanism

  13. Relationship of rate constant- temperature; Arrhenius equation K = Ae-Ea/RT ln k = -Ea/RT + ln A A : pre-exponational factor Ea : the energy of activation

  14. 1.6 Practical Kinetics • What is the time scale of the reaction? • Depending on the analytical devices • What species is chosen for the reaction?(reactants or products) • the best selection is that a method can provide concentrations precisely within 1-2 % error. • What kind of solvents is chosen for the reaction? • Solvent polarity, dielectric constant, hydrogen-bonding ability, donor capacity, and viscosity • What temperature is chosen for the reaction? • Temperature is controlled precisely with ± 0.2 ℃, reaction occurring temperature is in between -20 and 80 ℃

  15. 1.7 Pitfall • Identification of reactants and products • Solved by using spectroscopy devices • Purification of materials • Avoid oxygen(air) even if air-stable • Assuming that materials attended to properly • Analysis by means of a least-squares computer program • Signs of unexpected complications • Reaction proceed to completion, competing reaction, indication of an intermediate being formed, and so on.

  16. 1.8 Stoichiometric Equations, Elementary Reactions, Mechanisms, and Molecularity Co(NH3)5Br2+ + OH- = Co(NH3)5OH2+ + Br- v = k[Co(NH3)5Br2+][OH-] Co(NH3)4(NH2)Br+ → Co(NH3)4(NH2)2+ + Br- v = k[Co(NH3)4(NH2)Br+][Hg2+] Mercury(2+)-catalyzed reaction Co(NH3)5Br2+ + Nuc-= Co(NH3)5Nuc2+ + Br- v = k[Co(NH3)5Br2+] Nuc- = Cl-, N3-, etc.

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