Chapter 13: Chemical Kinetics
Chemical Kinetics
– The study of rates of chemical rxn
– analyzing the intermediates steps between initial to final steps
I.
Reaction Rate
– Change in concentration of one of the reactants or products divided by the time interval
A.
Average rate of disappearance of R = 
Δ
[R]/
Δ
t (R
Final
 R
Initial
)
B.
Average rate of formation of P =
Δ
[P]/
Δ
t
C.
Unique avg. rxn rate
•
Shows the relation between react and prod
•
Coefficients play an important role
EX: aA + bB
→
cC + dD
Unique avg. rxn rate = 1
Δ
[A]
= 1
Δ
[B]
= 1
Δ
[C]
= 1
Δ
[D]
a
Δ
t
b
Δ
b
c
Δ
t
d
Δ
t
Ex1:
2H
2
O
2
→
2H
2
O + O
2
in the interval of 60 sec, the [H
2
O
2
] decreased from 0.88M to 0.697M.
What is the
unique avg. rxn rate for all of the species involved in the rxn?
Unique rxn rate = 1
[0.697M – 0.88M]
= 1
[0.183M]
= 0.001542 M/s
2
60s
2
60s
Ex2:
The average rate of the reaction N
2
(g) + 3H
2
(g)
→
2NH
3
(g) over a certain period is reported as 1.15(mmol
NH
3
) L
1
∙
h
1
. a) What is the avg. rate over the same period in terms of the disappearance of H
2
? b) What is the
unique avg. rate?
a) 1.15(mmol NH
3
) L
1
∙
h
1
x 3 mmol H
2
_ = 1.73 (mmol H
2
) L
1
∙
h
1
2 mmol NH
3
b) 1.15(mmol NH
3
) L
1
∙
h
1
= 0.575 mmol
∙
L
1
∙
h
1
3
II.
Instantaneous Rate of Reaction

Slope of a tangent drawn to the graph concentration as a function of time

The rate decreases as the reaction proceeds
III.
Rate Laws

An expression for the instantaneous rxn rate in terms of the [] of a species at any time

Expression can contain react, prod, catalyst, etc., but usually only starting materials
A.
Rate constant –
k
(M/s)
•
Each rxn has its own constant
k
B.
Rate = constant x [concentration]
a
•
Rxn order is not related to coefficients
•
Ex: 2N
2
O
5
→
4NO
2
+ O
2
•
Rate Law =
k
[N
2
O
5
]
1
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C.
Firstorder Law
•
Rate =
k
[A]
1
•
If A is doubled (R =
k
[2A]), the rxn is 2x fast
•
Units of
k
: s
1
•
Formula: ln[A] = 
k
t + ln[A]
o
•
Plot ln[A] vs. time is a straight line
•
Slope = k (negative slope)
D.
Secondorder Law
•
Rate =
k
[A]
2
or Rate =
k
[A][B]
•
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 Spring '10
 johnson
 Chemical reaction

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