Chapter 13: Chemical Kinetics
CHE 124: General Chemistry II
Dr. Jerome Williams, Ph.D.
Saint Leo University
Overview
• Rate Laws & Reaction Orders
• Method of Initial Rates
The Rate Law
• Rate Law of a reaction is mathematical relationship
between rate of reaction and the concentrations of the
reactants
– and homogeneous catalysts as well
• Rate Laws are determined experimentally!!
• Rate of a reaction is directly proportional to the
concentration of each reactant raised to a power
• for the reaction aA + bB  products the rate law
would have the form given below
– n and m are called the orders for each reactant
– k is called the rate constant
Rate  k[A] [B]
n
m
Reaction Order
• the exponent on each reactant in the rate law is called
the order with respect to that reactant
• the sum of the exponents on the reactants is called the
order of the reaction
• The rate law for the reaction:
2 NO(g) + O2(g)  2 NO2(g)
Rate = k[NO]2[O2]
The reaction is second order with respect to [NO],
first order in [O2], and third order overall.
Sample Rate Laws
The bottom reaction is autocatalytic because a product affects the rate.
Hg2+ is a negative catalyst; increasing its concentration slows the reaction.
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Example: The rate equation for the reaction of NO with ozone is Rate =
k[NO][O3]. If the rate is 6.60 x 10−5 M/sec when [NO] = 1.00 x 10−6 M and
[O3] = 3.00 x 10−6 M, calculate the rate constant
Given:
Find:
Conceptual
Plan:
Relationships:
[NO] = 1.00 x 10−6 M, [O3] = 3.00 x 10−6 M,
Rate = 6.60 x 10−6 M/s
k, M−1s−1
Rate, [NO], [O3]
Rate = k[NO][O3]
Solve:
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k
Finding the Rate Law:
the Initial Rate Method
• Rate laws are determined experimentally
• The rate law shows how the rate of a reaction
depends on the concentration of the reactants
• Changing the initial concentration of a reactant
will therefore affect the initial rate of the reaction
then doubling the
initial concentration
of A quadruples
doubles
does
notthe
change
the
the initial
initial
reaction
reaction
rate
rate
if for the reaction
A → Products
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Rate = k[A]n
• If a reaction is Zero Order, the rate of the reaction is
always the same
– doubling [A] will have no effect on the reaction rate
• If a reaction is First Order, the rate is directly
proportional to the reactant concentration
– doubling [A] will double the rate of the reaction
• If a reaction is Second Order, the rate is directly
proportional to the square of the reactant
concentration
– doubling [A] will quadruple the rate of the reaction
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Determining the Rate Law When there Are
Multiple Reactants
• Changing each reactant will effect the overall
rate of the reaction
• By changing the initial concentration of one
reactant at a time, the effect of each
reactant’s concentration on the rate can be
determined
• In examining results, we compare differences
in rate for reactions that only differ in the
concentration of one reactant
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Example 13.2: Determine the rate law and rate constant for the
reaction NO2(g) + CO(g)  NO(g) + CO2(g)
given the data below
Comparing Expt #1 and Expt #2, the
[NO2] changes but the [CO] does not
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Example 13.2: Determine the rate law and rate constant for the
reaction NO2(g) + CO(g)  NO(g) + CO2(g)
given the data below
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Example 13.2: Determine the rate law and rate constant for the
reaction NO2(g) + CO(g)  NO(g) + CO2(g)
given the data below
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Example 13.2: Determine the rate law and rate constant for the
reaction NO2(g) + CO(g)  NO(g) + CO2(g)
given the data below
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Example 13.2: Determine the rate law and rate constant for the
reaction NO2(g) + CO(g)  NO(g) + CO2(g)
given the data below
n = 2, m = 0
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Example 13.2: Determine the rate law and rate constant for
the reaction NO2(g) + CO(g)  NO(g) + CO2(g)
given the data below
Substitute the
concentration
s and rate for
any
experiment
into the rate
law and solve
for k
Expt.
Initial
Initial
Number [NO2], (M) [CO], (M)
1.
2.
3.
4.
Rate

0.10
0.20
0.20
2
0.40
k[NO
]
2
0.10
0.10
0.20
0.10
for expt 1
0.0021
M
s


 k 0.10 M
2
0.0021 M s
1
1
k

0.21
M

s
0.01 M2
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Initial
Rate
(M/s)
0.0021
0.0082
0.0083
0.033
Practice – Determine the rate law and rate constant for the
reaction NH4+ + NO2− N2 + 2 H2O
given the data below
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