1. Kinetics
Lesson 5
PE Diagrams
Mechanisms

2. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products

3. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products

4. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products

5. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products

6. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products
Potential Energy

7. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products
Potential Energy

8. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products
  max
Potential Energy high
 low

9. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products
  max
Potential Energy high
 low
Kinetic Energy

10. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products
  max
Potential Energy high
 low
Kinetic Energy
high min low

11. Potential and Kinetic Energy Changes during a Collision
Endothermic 
Reactants Activated Complex Products
  max
Potential Energy high
 low
  Uphill- endothermic!
Kinetic Energy
high min low

12. Pick the slow and fast reaction

13. Pick the slow and fast reaction

14. Pick the slow and fast reaction
Fast- low Ea

15. Reaction Mechanisms
Consider the following reaction:
4 HBr(g) + O2(g) → 2H2O(g) + 2Br2(g)
Watch Simulation- set both temperature and mass to the highest setting. Set red to 1 and blue to four and remove the barrier. How often do all five particles collide?
There are five reactant particles.
If there are three or more reactant particles they will not likely react in one step.
They will react in a series of steps, which is a mechanism.

16. HBr(g) + O2(g) → HOOBr(g) slow HBr(g) + HOOBr(g) → 2HOBr(g) fast
Mechanism 1 .
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast PE
Reaction Path

17. Mechanism 1 Cancel out identical formulas to get the overall equation.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast PE
Reaction Path

18. Mechanism 1 Cancel out identical formulas to get the overall equation.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast PE
Reaction Path

19. Mechanism 1 Cancel out identical formulas to get the overall equation.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast PE
Reaction Path

20. HBr(g) + O2(g) → HOOBr(g) slow HBr(g) + HOOBr(g) → 2HOBr(g) fast
Mechanism 1 Intermediates are produced and then consumed and cross out from right (first) and left (second).
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast PE
Reaction Path

21. Mechanism 1 What is left is the overall reaction.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast PE
Reaction Path

22. Mechanism 1 What is left is the overall reaction.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g) PE
Reaction Path

23. HBr(g) + O2(g) → HOOBr(g) slow HBr(g) + HOOBr(g) → 2HOBr(g) fast
Mechanism 1 Step 1, which is the slow step, is called the rate determining step and has the highest activation energy.
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g) PE
Reaction Path

24. A potential energy diagram for this reaction might look like this.
Mechanism 1
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
A potential energy diagram for this
reaction might look like this. PE
Reaction Path

25. A potential energy diagram for this reaction might look like this.
Mechanism 1
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
A potential energy diagram for this
reaction might look like this. PE
Reaction Path

26. A potential energy diagram for this Mechanism has three humps
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
  highest
A potential energy diagram for this
Mechanism has three humps
The slow step has the highest Ea PE
Reaction Path Ea
Ea(rev)

27. The Rate Determining Step
The slowest step in the reaction mechanism is called the rate-determining step. It has the highest Ea. To increase the rate, you must increase the rate of this step. Increasing the rate of a fast step will not increase the rate of the overall reaction.

28. Identifying a Catalyst in a Mechanism
A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step than on the right side.

29. Identifying a Catalyst in a Mechanism
A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step than on the right side.
C →
→ C

30. Identifying a Catalyst in a Mechanism
A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step than on the right side.
C →
→ C

31. Identifying a Catalyst in a Mechanism
A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step than on the right side.
C →
→ C
An intermediate is produced and then consumed. It will be on the right side in an earlier step than on the left side.

32. Identifying a Catalyst in a Mechanism
A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step than on the right side.
C →
→ C
An intermediate is produced and then consumed. It will be on the right side in an earlier step than on the left side.
→ I
I →  

33. Identifying a Catalyst in a Mechanism
A catalyst is not consumed in the reaction. It is used in one step; it speeds up the reaction, and then is regenerated in a later step. A catalyst will cancel out but will be on the left side in an earlier step than on the right side.
C →
→ C
An intermediate is produced and then consumed. It will be on the right side in an earlier step than on the left side.
→ I
I →  

34. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst:
Intermediate:  

35. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst:
Intermediate:  

36. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst:
Intermediate:   C

37. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst:
Intermediate:   C

38. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst:
Intermediate:   C CD

39. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst:
Intermediate:   C CD

40. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation:
Catalyst: D
Intermediate:   C CD

41. Mechanism 2 1. A + B → C 2. C + D → CD 3. CD + E → ABE + D
2. C + D → CD
3. CD + E → ABE + D
Overall Equation: A + B E → ABE
Catalyst: D
Intermediate:   C CD

42. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction:
Intermediates: PE
Reaction Path

43. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction:
Intermediates: PE
Reaction Path

44. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: PE
Reaction Path

45. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl PE
Reaction Path

46. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH =
Ea (forward) =  
Ea (reverse) =
The enthalpy of Br
The enthalpy of BrCl PE
800
600
400
200
Reaction Path

47. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) =  
Ea (reverse) =
The enthalpy of Br  
The enthalpy of BrCl PE
Reaction Path
800
600
400
200

48. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) =  
Ea (reverse) =
The enthalpy of Br  
The enthalpy of BrCl PE
Reaction Path
800
600
400
200

49. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) = kJ  
Ea (reverse) =
The enthalpy of Br  
The enthalpy of BrCl PE
Reaction Path
800
600
400
200

50. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) = kJ  
Ea (reverse) = kJ
The enthalpy of Br  
The enthalpy of BrCl PE
Reaction Path
800
600
400
200

51. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) = kJ  
Ea (reverse) = kJ
The enthalpy of Br kJ  
The enthalpy of BrCl PE
Reaction Path
800
600
400
200

52. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) = kJ  
Ea (reverse) = kJ
The enthalpy of Br kJ  
The enthalpy of BrCl kJ PE
Reaction Path
800
600
400
200

53. Mechanism 4
The following mechanism shows how chlorine-containing fluorocarbons destroy the ozone layer. Ozone is O3 and O
1. CFCl3 → CFCl2 + Cl
2. Cl + O3 → ClO + O2
3. ClO + O → Cl + O2
Overall Reaction:
Intermediates:

54. Mechanism 4
The following mechanism shows how chlorine-containing fluorocarbons destroy the ozone layer. Ozone is O3 and O
1. CFCl3 → CFCl2 + Cl
2. Cl + O3 → ClO + O2
3. ClO + O → Cl + O2
Overall Reaction:
Intermediates:

55. Mechanism 4
The following mechanism shows how chlorine-containing fluorocarbons destroy the ozone layer. Ozone is O3 and O
1. CFCl3 → CFCl2 + Cl
2. Cl + O3 → ClO + O2
3. ClO + O → Cl + O2
Overall Reaction: CFCl O3 + O → CFCl O Cl
Intermediates:

56. Mechanism 4
The following mechanism shows how chlorine-containing fluorocarbons destroy the ozone layer. Ozone is O3 and O
1. CFCl3 → CFCl2 + Cl
2. Cl + O3 → ClO + O2
3. ClO + O → Cl + O2
Overall Reaction: CFCl O3 + O → CFCl O2+ Cl
Intermediates: Cl ClO

57. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD
Overall: A + D + E → ADE  

58. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD
Overall: A + D + E → ADE  

59. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD
Overall: A + D + E → ADE  
Put in step 3 so that you get the overall reaction

60. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD AD
Overall: A + D + E → ADE  
Put in AD to cancel

61. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD
AD + →
Overall: A + D + E → ADE  
Put in AD to cancel

62. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD
AD + E →
Overall: A + D + E → ADE  
Put in E

63. Mechanism 5 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → B + AD
AD + E → ADE
Overall: A + D + E → ADE  
Put in ADE

64. Mechanism 6 Determine the Missing Step of the Mechanism C + D → CD
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

65. Mechanism 6 Determine the Missing Step of the Mechanism C + D → CD
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

66. Mechanism 6 Determine the Missing Step of the Mechanism C + D → CD
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

67. Mechanism 6 Determine the Missing Step of the Mechanism C + D → CD
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

68. Mechanism 6 Determine the Missing Step of the Mechanism → C C + D → CD
→ C
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

69. Mechanism 6 Determine the Missing Step of the Mechanism → C C + D → CD
→ C
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

70. Mechanism 6 Determine the Missing Step of the Mechanism A + B → C
A + B → C
C + D → CD
CD + E → ABE + D
Overall: A + B + E → ABE  

71. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

72. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

73. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl Cl
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

74. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl Cl
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

75. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
Cl O3
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

76. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
Cl O3 → ClO O2
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

77. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
Cl O3 → ClO O2
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

78. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
Cl O3 → ClO O2
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

79. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
Cl O3 → ClO O2
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

80. Mechanism 7 Determine the Missing Step of the Mechanism
CFCl3 → CFCl2 + Cl
Cl O3 → ClO O2
ClO + O → Cl O2
Overall: O CFCl O3 → CFCl O Cl  

81. Discussion Topic Define Reaction Mechanism
Why is there a need of many steps?
What number of particles do not need many steps?
How to determine the rate-determining step?
What step’s rate can be changed to change overall rate of reaction?
What are intermediate and catalysts?
How to add steps to find overall reaction
How to find a missing steps from given steps and overall reaction?
How to find formula of activated complex of a step?

82. Questions:

83. Reaction mechanism PE diagram

84. Reaction mechansim PE diagram

85. Reaction mechanism PE diagram

86. Reaction mechanism PE diagram

87. A potential energy diagram for this Mechanism has three humps
HBr(g) + O2(g) → HOOBr(g) slow
HBr(g) + HOOBr(g) → 2HOBr(g) fast
2HOBr(g) +2HBr(g) → 2H2O(g)+ 2Br2(g) fast
4HBr(g) + O2(g) → 2H2O(g)+ 2Br2(g)
  highest
A potential energy diagram for this
Mechanism has three humps
The slow step has the highest Ea PE
Reaction Path Ea
Ea(rev)

88. What do you need to know? 1. ΔH of each step
2. Ea (forward) of each step
3. Ea (reverse) of each step
4. The enthalpy (PE) of reactants, intermediates, products, activated complex,
5. Fastest, slowest step
6. Slowest, fastest moving particles based of KE

89. Mechanism 3 Step 1 Br2 → 2Br Step 2 Br + OCl2 → BrOCl + Cl
Step 3 Br + Cl → BrCl
Overall Reaction: Br2 + OCl2 → BrOCl + BrCl
Intermediates: Br Cl
ΔH = kJ
Ea (forward) = kJ  
Ea (reverse) = kJ
The enthalpy of Br kJ  
The enthalpy of BrCl kJ PE
Reaction Path
800
600
400
200

90. Workbook page 30
Question 54-55