1. Reaction Rates and Equilibrium
CHAPTER 12
Reaction Rates and Equilibrium
12.1 Reaction Rates

2. In Chapter 11 How much product can be formed?
we discussed amounts of chemicals involved in a reaction:
How much product can be formed?
How much of a reactant is needed?
How much excess reactant remains?
Excess reactant
Limiting reactant

3. In Chapter 11 In Chapter 12 How much product can be formed?
we discussed amounts of chemicals involved in a reaction:
How much product can be formed?
How much of a reactant is needed?
How much excess reactant remains?
In Chapter 12
we will discuss the rate (speed) of a chemical reaction:
How fast is a reactant used up?
How fast does a product form?
What are factors that affect how fast a reaction occurs?

4. What is a rate? How is it measured?
The idea of rate
Some reactions are fast
Others are slow
C(s) + O2(g) → CO2(g)
4Fe(s) + 3O2(g) → 2Fe2O3(s)
What is a rate? How is it measured?

5. The idea of rate
Average speed traveled by a typical race horse:

6. The idea of rate The speed is not constant throughout the race
Average speed traveled by a typical race horse:
The speed is not constant throughout the race
Steeper slope:
higher speed
Most race horses finish at a faster speed than they start at

7. Can we measure time/distance?
Rate in chemistry
Consider the generic reaction: A → C
Can we measure time/distance?
How can we measure rates in chemistry?

8. Rate in chemistry Consider the generic reaction: A → C We monitor time
How can we measure rates in chemistry?

9. the number of A or C particles in the system
Rate in chemistry
Consider the generic reaction: A → C
We monitor time
We monitor
the number of A or C particles in the system
(or concentration)
How can we measure rates in chemistry?

10. Rate in chemistry Consider the generic reaction: A → C
How can we measure rates in chemistry?

11. At time 0 seconds A → C Beginning: High concentration of A
No C present
A → C

12. End:
High concentration of C
Low concentration of A
A → C

13. Why do the rates decrease over time?
Steeper slope: faster rate
“flatter” slope: slower rate
A → C

14. Because there are fewer molecules of A to convert into C
Why do the rates decrease over time?
Because there are
fewer molecules of A
to convert into C
A → C

15. Data for the reaction A → C
Average rate of consumption of A
Time (min)
Moles of A
Moles of C
1.00
0.00 10
0.74
0.26 20
0.54
0.46 30
0.40
0.60 40
0.30
0.70
Data for the reaction A → C
in a 1 L container

16. Data for the reaction A → C
Average rate of
consumption of A: –0.02 moles/(L·min)
Time (min)
Moles of A
Moles of C
1.00
0.00 10
0.74
0.26 20
0.54
0.46 30
0.40
0.60 40
0.30
0.70
Data for the reaction A → C
in a 1 L container

17. Data for the reaction A → C
Average rate of
consumption of A: –0.02 moles/(L·min)
Time (min)
Moles of A
Moles of C
1.00
0.00 10
0.74
0.26 20
0.54
0.46 30
0.40
0.60 40
0.30
0.70
Average rate of
formation of C:
Data for the reaction A → C
in a 1 L container

18. Data for the reaction A → C
Average rate of
consumption of A: –0.02 moles/(L·min)
Time (min)
Moles of A
Moles of C
1.00
0.00 10
0.74
0.26 20
0.54
0.46 30
0.40
0.60 40
0.30
0.70
Average rate of
formation of C: 0.02 moles/(L·min)
Data for the reaction A → C
in a 1 L container

19. Data for the reaction A → C
Average rate of
consumption of A: –0.02 moles/(L·min)
Time (min)
Moles of A
Moles of C
1.00
0.00 10
0.74
0.26 20
0.54
0.46 30
0.40
0.60 40
0.30
0.70
Average rate of
formation of C: 0.02 moles/(L·min)
1 mole A ~ 1 mole C
1:1 ratio
Data for the reaction A → C
in a 1 L container
The rate of consumption of A is equal to the rate of formation of C:
Molecules A are consumed as fast as molecules C are formed
Molecule A is consumed so its rate is a negative number

20. The decomposition of N2O5 follows the reaction
The rate of decomposition of N2O5 was measured after 25 s and was found to be 5.60 x 10–6 M/s. What is the rate of formation of NO2?
2N2O5(g) → 4NO2(g) + O2(g)

21. The decomposition of N2O5 follows the reaction
The rate of decomposition of N2O5 was measured after 25 s and was found to be 5.60 x 10–6 M/s. What is the rate of formation of NO2?
2N2O5(g) → 4NO2(g) + O2(g)
Asked: Rate of NO2 formation
Given: Rate of N2O5 decomposition
= 5.60 x 10–6 M/s
Relationships: 2 moles N2O5 ~ 4 moles NO2
Solve: Use mole ratios

22. The decomposition of N2O5 follows the reaction
2N2O5(g) → 4NO2(g) + O2(g)
Asked: Rate of NO2 formation
Given: Rate of N2O5 decomposition
= 5.60 x 10–6 M/s
Relationships:
2 moles N2O5 ~ 4 moles NO2
Solve: Use mole ratios
Remember

23. The decomposition of N2O5 follows the reaction
2N2O5(g) → 4NO2(g) + O2(g)
Asked: Rate of NO2 formation
Given: Rate of N2O5 decomposition
= 5.60 x 10–6 M/s
Relationships:
2 moles N2O5 ~ 4 moles NO2
Solve: Use mole ratios

24. The decomposition of N2O5 follows the reaction
2N2O5(g) → 4NO2(g) + O2(g)
Asked: Rate of NO2 formation
Given: Rate of N2O5 decomposition
= 5.60 x 10–6 M/s
Relationships:
2 moles N2O5 ~ 4 moles NO2
Solve: Use mole ratios
Answer: The rate of NO2 formation
is 1.12 x 10–5 moles/(L·s)

25. The decomposition of N2O5 follows the reaction
2N2O5(g) → 4NO2(g) + O2(g)
Asked: Rate of NO2 formation
Given: Rate of N2O5 decomposition
= 5.60 x 10–6 M/s
Relationships:
2 moles N2O5 ~ 4 moles NO2
Solve: Use mole ratios
Answer: The rate of NO2 formation
is 1.12 x 10–5 moles/(L·s)
Discussion:
The mole ratio tells us that the rate of NO2 formation (by moles) is twice the rate of N2O5 decomposition.
NO2 is formed twice as fast as N2O5 is decomposed (by moles).

26. Collision theory A + B → Products
Chemical reactions take place at the molecular level, where molecules of reactants are colliding with each other

27. Collision theory
The cue (white) ball collides with the orange ball and successfully sends it into the pocket

28. Collision theory But not all collisions are successful
Collision alone does not guarantee success

29. Collision theory But not all collisions are successful
Collision alone does not guarantee success
The same is true
in chemistry

30. Reaction profile
Reaction: A + B → C + D ∆H < 0
Reactants
Products

31. Reaction profile Reaction: A + B → C + D ∆H < 0
Energy is released as a result of the reaction
Reactants
Products

32. Reaction profile Reaction: A + B → C + D ∆H < 0
activation energy, Ea: the minimum amount of energy required for molecules to react.
Reactants
Products

33. Reaction profile Reaction: A + B → C + D ∆H < 0
activated complex, Ac: a high- energy state where bonds are being broken and reformed;
also referred to as the transition state.
Reactants
Products

34. Reaction profile Reaction: A + B → C + D ∆H < 0
activated complex, Ac
Ac is unstable and can:
go back to A + B (reactants)
proceed to C + D (products)
Reactants
Products

35. The reaction profile will look different
Reaction: A + B → C + D ∆H < 0
EnergyRectants > EnergyProducts
Consider the reverse reaction:
Reaction: C + D → A + B ∆H > 0
EnergyRectants < EnergyProducts
The reaction profile will look different

36. Reaction: C + D → A + B ∆H > 0
Energy is absorbed as a result of the reaction

37. A + B → C + D ∆H < 0 C + D → A + B ∆H > 0 Ea is larger!
Exothermic process
Endothermic process
Exothermic reactions tend to be more common than endothermic reactions because the energy barrier is lower

38. Based on this energy curve, this reaction is
an process
exothermic
endothermic

39. Based on this energy curve, this reaction is
an process
exothermic
endothermic

40. Based on this energy curve, this reaction is
an process
Can you place the following labels on this reaction profile?
Exothermic
Endothermic
Reactants
Products ∆H Ea Ac

41. Based on this energy curve, this reaction is
an process
Can you place the following labels on this reaction profile? Ac
Exothermic
Endothermic Ea
Products
Reactants ∆H

42. If there is enough energy to overcome the energy barrier, Ea
PROCEED
Reactant
Product
If there is enough energy to overcome the energy barrier, Ea
the activated complex becomes product

43. If there is not enough energy to overcome the energy barrier, Ea
GO BACK
Reactant
Product
If there is not enough energy to overcome the energy barrier, Ea
the activated complex become reactants again

44. Very few collisions result in the actual formation of products
GO BACK
Reactant
Product
Very few collisions result in the actual formation of products

45. Oxygen and carbon molecules collide all the time,
but the combustion of carbon will not start without an initial input of energy (such as a spark)
GO BACK
C(s) + O2(g)
CO2(g)
C(s) + O2(g) → CO2(g)

46. We saw earlier that the rates slow down over time
This is because there are fewer molecules of reactants present as the reaction progresses
A → C

47. The reaction rate is higher when more reactants are present
More collisions occur when the concentration of reactants is higher
More collisions means more reactions are possible
The reaction rate is higher when more reactants are present

48. What else can affect the rate
concentration of reactants affects
the reaction rate
What else can affect the rate
of a chemical reaction?
A → C

49. Temperature Energy barrier between reactants and products
Temperature is a measure of the average kinetic energy of molecules
The higher the kinetic energy,
the higher the number of molecules that successfully overcome the energy barrier

50. The reaction rate increases when the temperature increases
Energy barrier between reactants and products
The reaction rate increases
when the temperature increases

51. The reaction rate increases when the temperature increases
This is why food goes bad faster when it is warm
The reaction rate increases
when the temperature increases

52. Increased surface area leads to a higher reaction rate
Increased surface area means more particles are available for collisions
Increased surface area leads to a higher reaction rate A
More particles are exposed and
available to collide with other particles to have a reaction

53. Based on this energy curve, this reaction is
an process
Can you place the following labels on this reaction profile?
exothermic
endothermic Ea
Products
Reactants ∆H

54. Factors that affect the reaction rate:
Concentration of reactants
The higher the concentration of reactants, the higher the rate
Temperature
The higher the temperature, the higher the rate
Surface area
The higher the surface area, the higher the rate

55. Factors that affect the reaction rate:
Concentration of reactants
The higher the concentration of reactants, the higher the rate
Temperature
The higher the temperature, the higher the rate
Surface area
The higher the surface area, the higher the rate
Catalysts
This will be discussed in Section 12.4