Title: Lecture 60 Kinetics III
1Lecture 60 - Kinetics III
2A Model for Chemical Kinetics
A B C
3 things affect the reaction rate
1. Rate of collisions between A and B 2. Fraction
of collisions having correct orientation 3.
Fraction of collisions having sufficient
energy to cause reaction
31. Rate of Collisions
related to molecular speeds
higher concentrations more collisions
more reactions
42. Orientation of Collisions
e.g. 2 BrNO Br2 2 NO
Br
no reaction
52. Orientation of Collisions
- p steric factor
- fraction of collisions with correct
- orientation
- 0 lt p lt 1
63. Energy of Collisions
Transition State
O-N---Br---Br---N-O
Ea
Potential Energy
Reactants
BrNO BrNO
DE
Products
Br2 2 NO
Extent of Reaction
73. Energy of Collisions
- Ea is the activation energy
- If energy of collision gt Ea,
- products may be formed
- fraction of collisions with energy gt Ea is
8Reaction Rate
- rate p f collision rate
- p f Z AB
- but rate k AB
thus, k p Z f
-Ea/RT
pZe
(Arrhenius)
9Arrhenius Equation
-Ea/RT
k Ae
if Ea gt 0, k increases as T increases (more
energetic collisions)
Also, A µ T
10Arrhenius Equation
1. Knowing k vs. T, can find A, Ea 2. Knowing A,
Ea, can find k at any T
11Finding A and Ea
- re-cast the equation
- ln(k) ln(A) - Ea/RT
-Ea
1
ln(k)
ln(A)
R
T
y m x b
slope intercept
12Finding A and Ea
intercept ln(A)
slope -Ea/R
ln(k)
1/T
13for example,
- 2 N2O5(g) 4 NO2(g) O2(g)
- T, oC k, s-1 T, K ln(k) 1/T
- 20 2.0 x 10-5 293 -10.82 .00341
- 60 2.9 x 10-3 333 -5.84 .00300
14for example,
-5.84
x
slope -Ea/R, intercept ln(A)
x
ln k
x
-10.82
x
.00300
.00341
1/T
-5.84 - (-10.82)
slope
-1.2 x 104 K
(.00300 - .00341) K-1
15for example,
- but, slope -Ea/R
- thus, Ea - R (slope)
- -8.314 J K-1 mol-1 (1.2 x 104 K)
- 1.0 x 105 J/mol
16another example...
- O3 Cl OCl O2
- A 2.8 x 10-11 cm3 molecule-1 s-1
- Ea 2.1 kJ mol-1
- find k at -50oC
17O3 Cl OCl O2
-Ea/RT
k Ae
2.8 x 10-11 cm3 molecule-1 s-1 x
-2100 J/mol
8.314 J K-1mol-1 x 223 K)
e
9.02 x 10-12 cm3 molecule-1 s-1
18Catalysis
- A catalyst speeds up a reaction,
- but is not consumed itself
- The catalyst provides
- a different (and faster) mechanism
- for the reaction to proceed
19Catalysis
Cl
e.g. O3(g) O(g) ? 2 O2(g)
20Catalysis
O3 O ? 2 O2
O3 Cl ? ClO O2
Potential Energy
with a catalyst Ea is smaller
no catalyst
Extent of Reaction
21Heterogeneous Catalysis
- e.g. hydrogenation reactions
H
H
H
H
C C
H2(g)
H - C - C - H(g)
H
H(g)
H
H
ethylene
ethane
slow due to strong H-H bond!
22Heterogeneous Catalysis
Metal
23Heterogeneous Catalysis
Metal
1. Adsorption
24Heterogeneous Catalysis
Metal
1. Adsorption 2. Migration across surface
25Heterogeneous Catalysis
Metal
1. Adsorption 2. Migration across surface 3.
Reaction
4. Desorption