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Chemical Kinetics
Chapter 15
H2O2 decomposition in
an insect
H2O2 decomposition
catalyzed by MnO2
REACTION RATES
-
RR = D [P ] = D [R ]
Dt
P =products
Dt
R = reactants
Relative Rates
Reactant
2A g
-
4B + C
D [A ] = D [B ] = D [C ]
2D t
4D t
Dt
Rate Calculations
Collision Theory
NO
Collisions
Collisions
NO
YES
Energy
Collisions
Energy
Orientation
Factors Affecting RXN Rates
• *Nature of Reactants
• Temperature
• Concentration
• Surface Area/ Physical
state
• Catalysts
Simulation: RATE
MECHANISMS
A Microscopic View of Reactions
Sections 15.5 and 15.6
Mechanism: how reactants are converted
to products at the molecular level.
RATE LAW ---->
MECHANISM
experiment ----> theory
8
REACTION ORDER
In general, for
a A + b B --> x X
with a catalyst “C”
Rate = k
m
n
p
[A] [B] [C]
The exponents m, n, and p
• are the reaction order
• can be 0, 1, 2 or fractions
• must be determined by experiment!
More on Mechanisms
Reaction of
cis-butene -->
trans-butene is
UNIMOLECULAR only one reactant is
involved.
A bimolecular reaction
BIMOLECULAR — two
different molecules
must collide
--> products
Exo- or endothermic?
10
Collision Theory
Reactions require
(a) activation energy and
(b) correct geometry.
O3(g) + NO(g) ---> O2(g) + NO2(g)
1. Activation energy
2. Activation energy
and geometry
11
Mechanisms
O3 + NO reaction occurs in a single
ELEMENTARY step. Most others involve a
sequence of elementary steps.
Adding elementary steps gives NET reaction.
12
Mechanisms
Most rxns. involve a sequence of elementary
steps.
2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O
Rate = k [I-] [H2O2]
NOTE
1.
Rate law comes from experiment
2.
Order and stoichiometric coefficients not
necessarily the same!
3.
Rate law reflects all chemistry down to
and including the slowest step in multistep
reaction.
13
Mechanisms
Most rxns. involve a sequence of elementary steps.
2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O
Rate = k [I-] [H2O2]
Proposed Mechanism
Step 1 — slow HOOH + I- --> HOI + OHStep 2 — fast
Step 3 — fast
HOI + I- --> I2 + OH2 OH- + 2 H+ --> 2 H2O
Rate of the reaction controlled by slow step —
RATE DETERMINING STEP, rds.
Rate can be no faster than rds!
14
Mechanisms
15
2 I- + H2O2 + 2 H+ ---> I2 + 2 H2O
Rate = k [I-] [H2O2]
Step 1 — slow
Step 2 — fast
Step 3 — fast
HOOH + I- --> HOI + OH-
HOI + I- --> I2 + OH2 OH- + 2 H+ --> 2 H2O
Elementary Step 1 is bimolecular and involves Iand HOOH. Therefore, this predicts the rate law
should be
Rate  [I-] [H2O2] — as observed!!
The species HOI and OH- are reaction
intermediates.
16
Simulation:” Mechanisms
Rate Laws and
Mechanisms
17
NO2 + CO reaction:
Rate = k[NO2]2
Two possible
mechanisms
Two steps: step 1
Single step
Two steps: step 2
18
Ozone Decomposition Mechanism
2 O3 (g) ---> 3 O2 (g)
[O3 ]2
Rate = k
[O2 ]
Proposed mechanism
Step 1: fast, equilibrium
O3 (g) <--> O2 (g) + O (g)
Step 2: slow
O3 (g) + O (g) ---> 2 O2 (g)
19
Sovled problems: pg 144
20
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