8/31/2009
1
Chapter 12
Chemical Kinetics
12.1 Kinetics
`
The study of rates of reactions and reaction
mechanisms (e.g., how fast a reaction will go and what
steps it goes through)
`
What is rate?
`
Definition of reaction rate: the change in
concentration of a reactant or product over change in
time
→ Δ
[A] /
Δ
t
12.1 Factors that Affect Rate
`
Concentrations
`
Temperature
`
Pressure
`
Catalysts
`
Surface area
2NO
2
(g)
→
2NO (g) + O
2
(g)
`
How can rate be represented on a graph?
`
What happens to NO
2
over time?
NO?
O
2
?
`
How can their rates be expressed relative to each
other?
12.1 Graph of Reaction
12.1 Relative Rates
`
Rate of NO
2
consumption =
Rate of NO production =
2 × Rate of O
2
production
`
Then
t
]
O
[
t
2
NO]
[
t
2
]
NO
[
2
2
Δ
Δ
=
Δ
Δ
=
Δ
Δ
−
t
]
O
[
2
t
NO]
[
t
]
NO
[
2
2
Δ
Δ
=
Δ
Δ
=
Δ
Δ
−
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8/31/2009
2
12.1 Relative Rates
`
If NO
2
is disappearing at a rate of 4.2 x 10
5
M/s, what is
the rate of appearance of O
2
?
t
]
[O
t
]
NO
[
2
1
2
2
Δ
Δ
=
Δ
Δ
⎟
⎠
⎞
⎜
⎝
⎛
−
`
Write the rates of the species in the following reaction:
5Br

+ BrO
3

+ 6H
+
→
3Br
2
+ 2H
2
O
Group Quiz #1
`
According to the following
unbalanced
reaction, if the
rate of appearance of oxygen gas is 4.00 x 10
2
M/s, what
is the rate of disappearance of KClO
3
(g)?
KClO
3
(g)
→
KCl(g)
+
O
2
(g)
12.2 Rate Laws & Reaction Order
`
What happens to a chemical reaction over
time?
`
Over time, the change in concentration will
depend on the forward and reverse
i
reactions.
`
To simplify things, choose conditions before
products become significant
`
This allows us to examine rates using
only
reactants
12.2 Rate Laws
`
Reaction:
`
aA + bB
Æ
Products
`
Rate Law:
n
m
B
A
k
Rate
]
[
]
[
=
`
k is Rate Constant
`
m
& n are the
order of reactants
(NOT the
coefficient);
m & n must be determined
experimentally!
`
Reaction Order = sum of reactant orders
12.2 Rate Laws
`
Rate law
of the decomposition reaction of
NO
2
`
Rate =
k
[NO
2
]
m
`
k
=
rate constant
`
m
is
order of reactant
(NOT the coefficient);
m must be determined experimentally
`
Why do we want to determine the rate of a
reaction?
12.2 Types of Rate Laws
`
Differential Rate Law (or Rate Law)
`
rate depends on concentration
`
Integrated Rate Law
m
k
]
NO
[
t
]
[NO
Rate
2
2
=
Δ
Δ
−
=
`
rate depends on time