Chemical Kinetics

1. Chemical Kinetics Chapter 16

2. Kinetics • Reaction Rates • Factors affecting rate • Quantitative rate expressions • Determination • Factors • Models for Rates • Reaction Mechanisms • Effects of catalysts

3. Rates • Change in concentration of a reactant or product per unit time

6. Factors affecting rates • Nature of the reactants • State of subdivision/surface area • Concentration • Temperature • Catalysts

7. Reactants • Complexity • Bond strengths • Etc.

15. Concentrations as functions of time

16. Graph: Concentration vs. time

17. Average Rate • Change of concentration in a time interval

18. Average Rate • Slope of line between two points on the graph

19. Instantaneous rate • Slope of tangent line at a point on the graph

20. Instantaneous Rate 0.009 M 375 s

22. Initial Rate (t = 0)

23. Initial rate • Slope of tangent line at time 0 (y intercept)

24. Rate Laws • k = rate constant • m, n = order • rate = k[NO2]n

25. Introduction to Rate Laws • Reversible chemical reactions • Forward: • Backward: • Equilibrium:

26. Introduction • Dominant Reaction: • Rate Law: • k, k’: specific rate constant • n : order of reactant • can be zero, fractional, or negative

27. Method of Initial Rates • Unknown: k, m, n • Initial rate: instantaneous rate just after reaction is initiated

28. Initial Rates, NO2 decomposition

29. Order of Reaction • General: • Substituting: • Solution:

30. Rate constant • Rate 1 • 7.1 x 10-5 M s-1 = -k[0.01 M]2 • k = 0.71 M-1 s-1 • Rate 2 • 2.8 x 10-4 M s-1 = -k[0.02 M]2 • k = 0.70 M-1 s-1

31. You try

32. O2 + 2 NO  2NO2

33. Overall Order • Sum: 1 + 2 + 3 • = 6 • Overall order of reaction: 6

34. Types • Differential: • Rate dependence on concentration • Integrated: • Concentration dependence on time

35. First Order Reactions • For aA  products • Differential: • Integrated:

36. Half-life, first order reactions • Integrated law: • Half-life: • Half of initial reacted • [A]t = ½[A]0 • Independent of [A]0

39. Second Order Reactions • For aA  products • Differential: • Integrated:

40. Half-life, second order reactions • Integrated law: • Half-life: • Half of initial reacted • [A]t = ½[A]0 • Inversely proportional to [A]0

41. Zero Order Reactions • For aA  products • Differential: • Integrated:

43. Graphical Method • First order • Second order • Zero order • Straight line

44. First order • Plot: • ln[A] vs. time ln[A]0 slope = -k ln[A] time

45. Second order • Plot: • 1 vs. time • [A] slope = k 1 [A] 1 [A]o time

46. Zero order • Plot: • [A] vs. time [A]0 slope = -k [A] time

49. Summary • Conditions set so dominant forward reaction • Differential Rate Laws • rate as a function of concentration • method of initial rates • Integrated Rate Laws • concentration as a function of time • graphical method • Experimental data collection • Rate law types can be interconverted

50. Reaction Mechanism • Chemical equation: Summary • Mechanism: Series of elementary steps • Elementary Steps: Reactions with rate laws • from molecularity • Molecularity: Number of species that must • collide to produce reaction