Average rate of reaction:
C+2D
A+B
-D[A]
t
=
-D[B]
t
=
+D[C]
t
=
+D[D]
2t
The rate at which [A] and [B] decrease is equal to the rate
at which [C] increases and half the rate at which [D]
increases.
The rate of a reaction can be increased by:
• Increasing concentration
• Increasing temperature
• Decreasing particle size (increasing surface area
of reactants)
Collision Theory
First premise: Reactants must collide in order to react and
form products.
A2
+
B2
2 AB
Collision Theory
First premise: Reactants must collide in order to react and
form products.
A2
+
B2
2 AB
Second premise: Reactants must have the correct orientation
to form the products upon collision
Collision Theory
First premise: Reactants must collide in order to react and
form products.
A2
+
B2
2 AB
E < Ea
Second premise: Reactants must have the correct orientation
to form the products upon collision
Third premise: Reactants must have sufficient energy for the
collision to result in formation of products
Collision Theory
First premise: Reactants must collide in order to react and
form products.
A2
+
B2
2 AB
E > Ea
Second premise: Reactants must have the correct orientation
to form the products upon collision
Third premise: Reactants must have sufficient energy for the
collision to result in formation of products
Energy
Ea
Activated complex: an
unstable transition state
between reactants and
products. It can either
fall back down on the
reactant side or go on to
the product side.
DHrxn
Reaction progress
A catalyst speeds up the
rate of a reaction by
lowering the activation
barrier
Energy
Ea
Ea
DHrxn
Reaction progress
A catalyst speeds up the
rate of a reaction by
lowering the activation
barrier
Energy
Ea
DHrxn
Reaction progress
Reaction rate laws:
A
B
Rate = k[A]
This reaction is first
order in [A] and first
order overall
k = specific rate constant
•depends upon temperature
•unique for every reaction
•Indicates the probability of a successful conversion of
reactants to products
Reaction order is given by the sum of the exponents on
the molar concentrations in the rate law expression
General form of rate law:
aA + bB
Rate = k[A]m [B]n
products
• The only time m=a and n=b is when the reaction proceeds
in a single step, which is uncommon.
• The reaction order must be found experimentally by
comparing reactant concentrations.
Trial
[A]0
(M)
[B]0
(M)
Rate
(M/s)
1
0.100
0.100
2x10-3
2
3
0.200
0.200
0.100
4x10-3
0.200
16x10-3
Rate = k[A][B]2
Double [A] rate doubles
 First order in [A]
Double [B] rate quadruples
 Second order in [B]
Calculating rate constants
1. Determine the rate law.
Rate = k[A][B]2
2. Substitute in the reactant concentrations and measured
rates.
Rate = 2[A][B]2
3. Solve for k.
Trial
[A]0
(M)
[B]0
(M)
Rate
(M/s)
k
(M-2s-1)
1
0.100
0.100
2x10-3
2
2
0.200
0.100
4x10-3
2
3
0.200
0.200
16x10-3
2
Instantaneous rates vs. average rates:
• Average rates are found using:
D[A]
Dt
Example: [NO]0 = 0.200 M and [NO] = 0.150 M after 8 s.
What is the average rate?
0.150 M – 0.200 M
Avg. rate =
= -6.25 x 10-3 M/s
8s–0s
• Instantaneous rates are found using rate laws.
Example: What is the instantaneous rate if the reaction
above has the following rate law: Rate = (-0.2 M-1s-1) [NO]2
Rate = (-0.2 M-1s-1) [0.150 M]2 = -4.5 x 10-3 M/s