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Chapter 13

Chapter 13. Kinetics. Overview. Rates of Chemical Reaction Pathways or Mechanisms of Reaction. Factors affecting Rate. [Reactants] recall, [ molarity] Temp. Surface Area Catalyst define catalyst vs inhibitor. Rate, defined. A  C Δ [A] / sec

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Chapter 13

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  1. Chapter 13 Kinetics

  2. Overview • Rates of Chemical Reaction • Pathways or Mechanisms of Reaction

  3. Factors affecting Rate • [Reactants] recall, [ molarity] • Temp. • Surface Area • Catalyst define catalyst vs inhibitor

  4. Rate, defined A  C Δ [A] / sec Rate of disappearance of A with respect to time ( or may monitor appearance of product ) ex. 13.1 page 531

  5. Rate Law A + B  C + D • Form rate = k [A]m[B]n • Derived from experiment • m,n are coeffs of slow step • Exponent value ~ “order” (examples) • k is “rate constant”

  6. Units for k, rate constant M (1- overall order) s -1 Where: M, molarity s, seconds See page 536, bottom right summary

  7. Rate Law Determination Method of “Initial Rates” • RE:lab • Example: 2 NO + Cl2 2NOCl data given in class

  8. Integrated Rate Law • Finding [A]t at a later time, t, if [A]0 is initially known ln [A]t / [A]0 = - kt for 1st order Rearrange terms ; straight line form; graphically determine – see p. 539

  9. Half-Life of Reaction, t1/2 • Defined • Zero order: t ½ = [A]0 / 2k • Ist order: t ½ = 0.693/ k • 2nd order: t ½= 1/ k[A]0 Summary table 13.4, page 546

  10. Collision Theory • Rate depends on: # of collisions % effective (% incr, as temp incr ) • Ea defined

  11. Collision Theory, contd • Transition state • Activated complex • Profiles draw & compare ΔHes and Eas for exo and endo cases • Notes: ΔH = Eafw – Eabckrate increases as T increases for both fw and bck rxns

  12. Arrhenius Equation k = A e –Ea/RT Now, lnk = –Ea / RT + lnA lnk = –Ea / R (1/T) + ln A straight line; find Ea graphically Or ln k2/k1 = Ea / R { 1/ T1 - 1/ T2 } Where T is Kelvin

  13. Mechanisms • Steps; path • Steps are reversible • “slow step” is rate determining if rate = k [A]m[B]n m,n are coeffs of slow step in mechanism • Terms: “elementary reaction” is a step unimolecular, bimolecular, etc

  14. Mechanisms, contd. • Intermediates formed/ consumed • Catalysts regeneratedskim read mechanism proposals p. 555

  15. Catalysts • Lower Ea • New mechanism • New slow step • Homogeneous catalysis • Heterogeneous catalysissee profiles, page 558

  16. Enzymes • Biological catalysts, E • Lock and key model • Active site • Substrate, S

  17. Factors Affecting Enzyme Activity • [S] • [E] • pH • T see profiles: p. 561 [E] = con. & [S] changes [S] = con;xs & [E] changes

  18. Inhibitors • Destroy lock and key “fit” • Active site is no longer “active” • Example, lipitor

  19. End Ch. 13 • Homework • Exam # 1 Ch. 6 & 13

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