Chapter 17
Reaction Kinetics
17-1 The Reaction Process
Can you
remember
the first time
you ever
made a
friend?
What had to
happen
before the
friendship
could
begin?
Mutual
Friend
Accidentally
Bumped
into each
other
How did
you
meet?
Eye
Contact
Collision Theory
•
In order for a reaction to occur particles
must collide in:
1. A specific orientation and
2. with enough energy
Activation Energy
• The amount of energy required for a reaction to
occur
Activation Energy
• Activation energy - the amount of
energy the particles must have when
they collide to force a reaction to occur.
Activation
Energy
Reactants
Products
∆H will be
negative since
energy has left
the system
Reaction Pathways
The products
have less
energy than the
reactants. The
rxn released
energy (heat) =
exothermic
∆H will be positive
since energy has
been added to the
system
Reaction Pathways
The products
have more
energy than
the reactants.
The rxn
absorbed
energy (heat)
=
endothermic
Practice
• Draw and label the energy diagram for a
reaction in which ΔE = 30 kJ/mol, Ea = 40
kJ/mol. Place reactants at energy level
zero. Indicate determined values of
ΔEforward, ΔEreverse & Ea’
Reaction Mechanisms
• Step-by-step sequence of rxns in order to obtain
a final product
Proposed Mechanism for Ozone Depletion via Free
Chlorine Atoms Created by Decomposition of CFCs
Step 1) Cl + O3 → ClO + O2
Step 2) 2 ClO → ClOOCl
Step 3) ClOOCl → ClOO + Cl
Step 4) ClOO → Cl + O2
Mechanisms
overall rxn
Intermediates
Mechanisms
overall rxn
Rate Determining Step
Slow
Fast
Fast
Fast
Catalysts vs. Intermediates
overall rxn
Catalysts appear 1st as
a reactant and then as
a product during a
mechanism.
Intermediates appear
1st as a product and
then as a reactant
during a mechanism.
Chapter 17
Reaction Kinetics
17-2 Reaction Rate
How can we increase the rate
of a reaction?
1.
2.
3.
4.
5.
Increase Surface Area
Increase Temperature
Increase Concentration
Increase in Pressure
Add a Catalyst
Surface Area
• Increase the surface area allows for a greater
chance for effective collision
Temperature
• An increase in temperature will cause
particles to move at a higher velocity resulting
in more effective collisions
Concentration
• An increase in concentration will also cause an
increase in the chance that effective collisions
will occur
Pressure
• Increasing the pressure of a gas system will
cause more frequent collisions
Catalysts
• Adding a catalyst lowers the amount of
activation energy required
Catalysts
Reactants
Catalyst
Rate Laws
• An equation that relates the rxn rate and the
concentration of reactants
Rate Determining Step
Slow
Rate = k[HBr][O2]
Rate Laws
• If no mechanism is given, then…
2H2 + 2NO  N2 + 2H2O
Rate = k[H2]2[NO]2
Rate Orders
1st order
ln [reactants]
[reactants]
0 order
1/[reactants]
• 0, 1st and 2nd order rates
• Order is dependent upon what will yield a
straight line
2nd order
Rate Orders
For Individual Components:
• 1st order: reaction rate is directly
proportional to the concentration of that
reactant
• 2nd order: reaction rate is directly
proportional to the square of that reactant
• 0 order: rate is not dependant on the
concentration of that reactant, as long as it
is present.
Rate Orders
For Overall Order:
• Overall reaction orders is equal to the sum
of the reactant orders.
• Always determined experimentally!
Calculating for k
A + 2B  C
Rate = k[A][B]2
Experiment
Initial [A]
Initial [B]
Rate of
Formation of
C
1
0.20 M
0.20 M
2.0 x 10-4
M/min
2
0.20 M
0.40 M
8.0 x 10-4
M/min
3
0.40 M
0.40 M
1.6 x 10-3
M/min
What is the value of k, the rate
constant?
Calculating for k
Experiment
Initial [A]
Initial [B]
Rate of
Formation
of C
1
0.20 M
0.20 M
2.0 x 10-4
M/min
2
0.20 M
0.40 M
8.0 x 10-4
M/min
3
0.40 M
0.40 M
1.6 x 10-3
M/min
Rate = k[A][B]2
2.0 x 10-4 = k[0.20][0.20]2
2.0 x 10-4 = k(0.008)
k = 2.50 x 10-2 min-1 M-2
Practice
1. In a study of the following reaction:
2Mn2O7(aq) → 4Mn(s) + 7O2(g)
When the manganese heptoxide concentration was changed
from 7.5 x 10-5 M to 1.5 x 10-4 M, the rate increased from 1.2 x
10-4 to 4.8 x 10-4. Write the rate law for the reaction.
2. For the reaction:
Rate = k[Mn2O7]2
A+B→C
When the initial concentration of A was doubled from 0.100 M
to 0.200 M, the rate changed from 4.0 x 10-5 to 16.0 x 10-5.
Write the rate law & determine the rate constant for this
Rate = k[A]2
reaction.
Constant = 4.0 x 10-3 M/s
More Practice
3. The following reaction is first order:
CH3NC(g) → CH3CN(g)
The rate of this reaction is 1.3 x 10-4 M/s when the reactant
concentration is 0.040 M. Predict the rate when [CH3NC] =
0.025.
New Rate = 8.1 x 10-5 M/s
4. The following reaction is first order:
(CH2)3(g) → CH2CHCH3 (g)
What change in reaction rate would you expect if the pressure
of the reactant is doubled?
An increase by a factor of 2
Even More Practice
5. The rate law for a single step reaction that forms one product, C
is R = k[A][B]2. Write the balanced reaction of A & B to form C.
A + 2B → C
6. The rate law of a reaction is found to be R = k[X]3. By what
factor does the rate increase if the concentration of X is tripled?
The rate will increase by a factor of 27
7. The rate of reaction, involving 2 reactants, X & Z, is found to
double when the concentration of X is doubled, and to
quadruple when the concentration of Z is doubled. Write the
rate law for this reaction. R = k[X][Z]2